QD 

5  5  Tt— ~ — — ™— — 

0  2  * .ECTROCHEMICAL 

EXPERIMENTS 
^1  OETTEL 


UC-NI 


277    7M7 


GAR  R SMITH 


LIBRARY 

OF  THE 

UNIVERSITY  OF  CALIFORNIA. 


Class 
\ 


INTRODUCTION 


TO 


ELECTROCHEMICAL 
EXPERIMENTS 

BY 

DR.    FELIX    OETTEL 


TRANSLATED 
(WITH  THE  AUTHOR'S  SANCTION) 

BY 

EDGAR    F.    SMITH 

)F   CHEMISTRY    IN   THE   UNIVERSITY    OF    PENNSYLVANIA 


-DIVERSITY 


OF 


J^Smb  {TwentE*sii  Illustrations 


PHILADELPHIA 
P.    BLAKISTON,    SON    &    CO. 

1012    WALNUT    STREET 
I897 


Q'D 


COPYRIGHT,  1897,  BY  P.  BLAKISTON,  SON  &  Co. 


PRESS   OF  WM.   F.  FELL  &.  CO., 

I22O-24  SANSOM  ST., 

PHILADELPHIA. 


PREFACE. 


THE  purpose  of  this  little  volume  is  to  furnish  tech- 
nical chemists  and  all  persons  interested  in  the  ap- 
plications of  electricity  to  chemical  manufacture  with 
a  concise  guide,  containing  in  a  compact  form  all 
that  is  essential  for  the  comprehension  and  solution 
of  problems  arising  in  this  comparatively  new  field  of 
chemical  investigation.  That  it  has  in  a  measure 
fulfilled  its  mission  is  evidenced  by  the  hearty  recep- 
tion and  favorable  criticism  accorded  it  in  its  German 
home,  and  by  its  translation  into  Russian.  That 
it  may  prove  equally  helpful  to  all  its  English-speak- 
ing readers  is  the  hope  of  the  translator,  to  whom  it 
is  a  pleasure  to  acknowledge  indebtedness  to  Prof. 
George  F.  Barker  for  advice  and  assistance  in  the 
preparation  of  the  text  and  correction  of  the  proof 
sheets. 


TABLE  OF  CONTENTS. 


PAGE 

A.     SOURCE,  MEASUREMENT,  AND^REGULATION  OF  CURRENT,  .  9 
Sources  of  Current. 

Galvanic  Batteries — Primary  Batteries 10 

Arrangement  of  Batteries,       14 

Storage  Cells — Accumulators, 20 

Thermopiles, 25 

Dynamos, 27 

Rules  for  Arrangement  and  Working  of  a  Small  Dynamo,  34 

Current  Measurement, 37 

Silver  Voltameter, 38 

Copper  Voltameter, .    .  39 

Oxyhydrogen  Voltameter, 4° 

Tangent  Galvanometer, 43 

Torsion  Galvanometer, 47 

Technical  Measuring  Apparatus, 49 

Ampere-hour'Meter, 51 

Measurement  of  Pressure, .    .  53 

Regulation  of  Current, 57 

B.     ARRANGEMENT  OF  EXPERIMENTS. 

Vessels, 63 

Diaphragms, 65 

Electrodes, 68 

Conductors,      71 

Electrolyte,      73 

Sketch  of  Arrangement  of  Experiment, 75 

vii 


Vlll  TABLE    OF    CONTENTS. 

C.  PHENOMENA  OBSERVED  IN  ELECTROLYSIS.  PAGE 

Decomposition  Pressure.      Polarization  Current, 77 

Law  of  Faraday.     Current  Efficiency, 88 

Transference  of  Ions,      . 92 

Current  Density,      94 

Working  Pressure, 96 

D.  PRELIMINARY  EXPERIMENTS  OF  AN  ELECTROLYTIC  PRO- 

CESS,       98 

E.  .CALCULATION  OF   NECESSARY   POWER.     CHOICE  OF  DY- 

NAMO,    100 

F.  PRACTICAL  PART. 

1.  Construction  and  Calibration  of  a  Tangent  Galvanometer,  104 

2.  Calibration  of  a  Galvanometer  by  Means  of  a  Shunt,  .    .  107 

3.  Construction  of  Simple   Instrument   for  Measuring  Pres- 

sure,           ...  Ill 

4.  Calculation  for  and  Construction  of  a  Regulating  Resist- 

ance,        115 

5.  Working  of  an  Arsenical  Copper  Liquor, 118 

6.  Arrangement  for  Electrochemical  Analysis, 134 

G.  TABLES. 

I.   Electrochemical    Equivalents   of  the  More  Important 

Elements, 141 

II.   Thermochemical  Data, 142 

III.  Wire  Resistances, 144 


INTRODUCTION 


TO 


Electrochemical  Experiments. 


A.    SOURCE,  MEASUREMENT,  AND  REGU- 
LATION OF  THE  CURRENT. 

SOURCES  OF  THE  CURRENT. 

Four  sources  may  be  considered  :  ordinary  galvanic 
batteries  (primary  batteries),  storage  cells  or  accumu- 
lators, thermopiles,  and  dynamo  machines.  The  first 
are  most  readily  obtained,  but  constitute  a  rather  in- 
complete adjunct.  Storage  cells  are  decidedly  more 
advantageous,  so  far  as  regards  constancy  of  current 
and  cleanliness  in  handling.  Those  who  have  em- 
ployed this  source  of  electric  energy  return  to  the 
ordinary  battery  only  in  cases  of  absolute  necessity. 
Storage  cells  may  be  charged  either  by  dynamos  or 
by  thermopiles.  The  latter  were,  for  a  time,  deemed 
worthless,  but  since  their  improvement  in  construc- 
tion have  again  come  into  favor.  They  are  to  be 
recommended  for  experimental  work  on  a  small  scale, 

*  9 


IO         ELECTROCHEMICAL  EXPERIMENTS. 

and  for  analytical  purposes,  but  are  only  available 
when  illuminating  gas  can  be  had.  A  small  dynamo 
is  the  best  source  of  current  for  various  practical 
experiments. 

In  undertaking  an  electrochemical  investigation,  the 
most  rational  beginning  is  the  selection  of  the  proper 
working  conditions  for  the  experiment,  aside  from 
batteries  or  accumulators.  With  the  data  and  experi- 
ence thus  gained,  the  next  step  is  the  arrangement  of 
a  small  plant  provided  with  a  dynamo.  Difficulties, 
as  a  rule,  now  appear;  these  are  generally  of  a  con- 
structive nature.  When  they  have  been  surmounted, 
and  the  miniature  experimental  plant  serves  its 
purpose  properly,  calculations  for  some  definite  pro- 
cess may  next  engage  the  attention  of  the  experi- 
menter. 

After  this  brief  enumeration  of  the  different  sources 
of  electric  energy  and  their  general  application,  a 
more  detailed  description  of  them  may  be  given. 

PRIMARY    BATTERIES    (GALVANIC    CELLS). 

Cells  of  this  class,  designed  for  the  execution  of 
electrochemical  experiments,  should  furnish  a  strong 
and  constant  current.  Many  forms  have  been  de- 
vised, but  there  is  not  one  which  fully  meets  these 
conditions,  hence  there  can  be  no  purpose  in  entering 
into  a  detailed  description  of  each  variety.  Two  may 
be  mentioned — the  Bunsen  cell  and  the  Daniell  cell. 

The  Bunsen  cell  is    the  zinc-carbon    combination. 


SOURCES    OF    THE    CURRENT.  I  I 

It  consists,  usually,  of  a  glass  jar  in  which  there 
is  a  heavy,  amalgamated  zinc  cylinder  ( —  pole). 
Within  the  latter  stands  a  porous  cup.  This  con- 
tains concentrated  nitric  acid,  in  which  is  immersed 
a  bar  of  hard  gas-carbon.  The  glass  jar  contains 
dilute  sulphuric  acid  (1:20).  The  action  of  the 
current  causes  a  reduction  of  the  nitric  acid,  and 
fumes  of  nitrogen  dioxide  arise,  making  it  necessary 
to  keep  the  batteries  in  a  good  draught  chamber. 
This  inconvenience  may  be  partly  obviated  by  drop- 
ping into  the  cup,  from  time  to  time,  chromic  acid  or 
potassium  bichromate.  The  electromotive  force  of 
the  cell  is  1.8  V.  At  first  the  current  from  it  is 
very  strong,  but  it  grows  considerably  less  in  the 
course  of  a  few  hours.  This  form  of  battery  is  very 
well  adapted  for  experiments  requiring  a  strong  cur- 
rent for  a  relatively  short  time,  and  where  high  pres- 
sures are  necessary.  It  is  not  suited  for  experiments 
extending  through  a  period  of  days. 

The  following  points  should  be  observed  when 
using  this  battery.  The  zinc  must  be  well  amalga- 
mated, otherwise  a  tumultuous  evolution  of  hydro- 
gen will  occur,  leading  to  a  rapid  consumption  of  the 
zinc.  In  amalgamating,  first  dip  the  zinc  cylinder 
into  very  dilute  sulphuric  acid,  then  pour  mercury 
over  it,  and  distribute  the  latter  with  a  brush.  The 
clay  cup  should  not  be  so  porous  that  much  nitric 
acid  can  reach  the  zinc.  In  arranging  the  battery, 
the  sulphuric  acid  is  first  introduced,  and  when  the 


12  ELECTROCHEMICAL    EXPERIMENTS. 

porous  cup  has  become  thoroughly  permeated  with 
it,  the  nitric  acid  is  introduced  into  the  cup.  As  re- 
gards the  carbon  bars,  it  may  be  said  that  those  from 
natural  retort  carbon  are  superior  to  those  made 
from  pressed  carbon.  To  prevent  them  from  absorb- 
ing nitric  acid,  which  would  eventually  reach  and 
destroy  the  metallic  binding-screw  attachments  at 
their  exposed  ends,  the  bars  should  be  heated  or 
dried  thoroughly  and  the  ends  then  immersed  in 
melted  paraffin,  the  excess  of  the  latter  being  re- 
moved with  a  brush.  The  binding  screws,  in  union 
with  the  battery  poles,  should  be  clean  and  bright ; 
the  other  parts  may  be  covered  with  an  asphalt  paint, 
and  in  this  way  be  protected  from  acid  vapors.  The 
current  may  be  conducted  from  the  battery  according 
to  the  experiment  by  a  stout  copper  wire  (not  less  than 
one  mm.  in  diameter).  This  is  sometimes  rolled  into 
spirals,  although  there  is  really  no  necessity  for  so 
doing. 

The  Daniell  cell  possesses  less  electromotive  force 
but  greater  constancy  than  the  Bunsen  cell.  It  con- 
sists of  amalgamated  zinc  in  dilute  sulphuric  acid, 
and  copper  in  a  saturated  copper  sulphate  solution, 
the  two  liquids  being  separated  by  a  porous  cup. 
The  sheet  of  copper  serving  as  the  positive  pole  is 
cylindrical  in  shape,  and  perforated.  A  copper  wire 
is  welded  to  it.  In  order  to  maintain  a  concentrated 
solution,  a  perforated  bottom  is  sometimes  placed  in 
the  upper  portion  of  the  jar.  Crystals  of  copper 


SOURCES    OF    THE    CURRENT.  13 

sulphate  are,  from  time  to  time,  placed  upon  it.  If 
zinc  be  on  the  exterior  and  copper  be  placed  in  the 
porous  cup,  the  combination  will  prove  more  energetic 
than  with  the  reverse  condition.  The  pressure  of 
such  a  cell  will  be  about  1.05  V.  The  nature  of  the 
porous  cup  affects  the  efficiency  of  a  battery  very 
much.  If  it  be  too  dense,  the  internal  resistance  of 
the  cell  will  be  too  great.  If  it  be  too  porous,  then 
the  copper  solution  will  penetrate  to  the  zinc  and 
the  latter  will  rapidly  become  covered  with  a  film  of 
copper,  in  consequence  of  which  the  action  of  the 
cell  will  be  much  diminished. 

In  testing  a  porous  cup,  fill  it,  when  perfectly  dry, 
with  water.  It  should  be  wet  throughout  within  a 
few  minutes,  but  the  water  should  pass  through  it  very 
slowly.  It  is  a  good  rule  to  reject  all  porous  cups 
which,  shortly  after  they  have  been  moistened,  show 
drops  of  water  on  their  external  surface. 

Another  form  of  Daniell  cell,  arranged  for  con- 
tinuous use,  consists  of  a  hollow  zinc  cylinder,  not 
amalgamated,  standing  in  a  concentrated  zinc  sul- 
phate solution.  In  the  porous  cup  there  is  a  copper 
cylinder  immersed  in  a  solution  of  copper  sulphate. 
By  use,  the  latter  loses  color,  indicating  that  it  is 
necessary  to  add  crystals  of  blue  vitriol.  From  time 
to  time  the  zinc  solution  is  siphoned  off,  an  equal 
volume  of  water  being  added  to  prevent  the  separation 
of  crystals  of  zinc  sulphate.  Once  or  twice  every 
week  the  zinc  cylinder  should  be  taken  out  and 


14  ELECTROCHEMICAL    EXPERIMENTS. 

cleaned.  An  arrangement  of  this  sort  consumes 
very  little  zinc,  but  has  a  greater  internal  resistance. 
Its  pressure  does  not  exceed  0.9-0.95  V. 

The  disagreeable  deposit  of  salts  on  the  edges  and 
over  the  sides  of  the  cells  can  be  lessened  by  employ- 
ing porous  cups  with  glazed  edges,  or  by  paraffining 
the  portion  of  the  cup  extending  beyond  the  liquid. 
It  can  not,  however,  be  wholly  overcome.  When  the 
battery  is  disconnected,  the  carbons  and  the  porous 
cups  should  be  thoroughly  washed,  and  put  away 
when  dry.  If  the  washing  be  omitted,  the  cups  will 
be  cracked — or  at  least  be  damaged  to  a  marked 
decree — by  the  salts  which  crystallize  out. 

When  a  battery  or  cell  is  to  be  again  put  together, 
the  cup  must  first  be  thoroughly  saturated  with  the 
zinc  sulphate  solution,  and  the  blue  vitriol  solution 
then  be  introduced  into  it.  If  the  cup  be  not  dry, 
but  moist  with  water,  it  will,  in  consequence,  yield  but  a 
feeble  current  until  its  walls  are  filled  by  diffusion 
with  the  better  conducting  vitriol  solution. 

ARRANGEMENT    OF    CELLS. 

Having  a  number  of  cells,  it  is  possible  to  arrange 
them  in  three  different  ways  : 

(a)  Parallel,  when    all    the    zinc    poles    are    con- 
nected with  one  another,  and  the  copper  poles  in  like 
manner  (Fig.  i). 

(b)  In  scries,  when  each  zinc  pole  is  connected  with 
the  copper  pole  of  the  adjacent  cell  (Fig.  2). 


SOURCES    OF    THE    CURRENT.  15 

(c)  In  groups  (inixed  arrangement] ;  an  equal  number 
of  cells  are  united  into  groups  and  the  latter  then 
arranged  as  individual  cells.  The  arrangement  will 


uu/ 


FIG. 


be  different,  depending  upon  whether  that  of  (a) 
Fig.  3  or  (<£)  Fig.  4  is  observed  within  the  group. 
The  first  is  preferable,  because  slight  defects  in  the 
individual  cells  are  less  disturbing. 


FIG.  2. 


When  should  the  one  or  the  other  combination  be 
chosen  ?  In  answering  this  question,  the  following 
points  should  be  remembered : 


i6 


ELECTROCHEMICAL  EXPERIMENTS. 


(1)  The  maximum   work  of  a  battery  is  obtained 
when  the  resistance  in  the  outer  circuit  is  equal  to 
the  total  resistance  of  the  battery. 

(2)  In  the  parallel  arrangement  of  cells,  the  electro- 
motive force  of  the  battery  is  not  altered.     The  inter- 


FIG.  3. 


FIG. 


nal  resistance  is,  however,  diminished  directly  accord- 
ing to  the  number  of  cells. 

(3)  When  cells  are  arranged  in  series,  both  the 
pressure  and  the  resistance  of  the  individual  cells  are 
increased  in  the  sum  total. 

If  the  electromotive  force  of  a  cell  be  represented 


SOURCES    OF    THE    CURRENT.  I/ 

by  e,  and  its  internal  resistance  by  w,  then  a  battery 
of  n  cells  arranged  parallel  would  have 

the  electromotive  force  e 

and 

the  internal  resistance  — . 

n 

A  battery  with  cells   in  series  would,  on  the  con- 
trary, have 

the  electromotive  force  ;/.  e 

and 
the  internal  resistance  n.  w. 

With  a  battery  of  n  cells  in  series,  composed  of  a 
elements  parallel,  each  group  would  have 

the  electromotive  force  e 

and 

the  internal  resistance  — , 


so  that  in  the  entire  battery  of  n  such  groups  there 
would  be 

the  electromotive  force  n.  e 

and 

the  internal  resistance  n. 

a 

According  to  Ohm's  law,  the  current  strength  in  a 
circuit  is  equal  to  the  electromotive  force  divided  by 
the  total  resistance.  The  latter  equals  the  sum  of  the 
internal  resistance  Wj  of  the  battery  and  the  resistance 


I  8         ELECTROCHEMICAL  EXPERIMENTS. 

Wa  of  the  external  circuit,  so  that  the  current  strength 
I  may  be  expressed  by  the  formula 

E 

=   Wi    +    Wa  ' 

This  formula  gives  two  possibilities  for  the  increase 
of  current:  either  by  increase  of  the  numerator  or  the 
diminution  of  the  denominator.  The  first  follows 
from  the  arrangement  of  cells  in  series  (increase  of 
electromotive  force),  the  second  from  their  parallel 
arrangement  (reduction  of  Wi).  Which  course  should 
be  pursued  depends  upon  whether  Wa  is  large  or 
small  in  proportion  to  W{.  Several  examples  will 
serve  to  demonstrate  that  in  cases  of  great  external 
resistance  a  series  arrangement  of  the  cells  is  the 
proper  arrangement,  while  with  low  external  resistance 
the  parallel  arrangement  should  be  chosen. 

Examples : 

The  E.  M.  F.  of  a  cell  is  1.05  V. 

Internal  resistance  of  a  cell  is  0.5  $. 

(a)  External  resistance  is  10  Q. 

Current  strength  with  various  combinations  : 

1.05 

1  cell, I  =  -  --=0.10  amp. 

0.5  +  10 

2.        I.O5 

2  cells  in  series,  ....!=—  =0.19  amp. 

2.     0.5   f  10 

.    4.     1.05 

4  cells  in  series,  .    .    .    .  I  =  —  —  =  o.  15  amp. 

4.     0.5  -f  10 


SOURCES   OF   THE   CURRENT. 

Reverse  : 

1.05 

2  cells  in  parallel,  .    .    .  I  =  —  -  =  o.  IO2  amp. 


1.05 
4  cells  in  parallel,  .    .    .  I  =  —  =  o.  104  amp. 

*-+  10 

(b)  External  resistance  .-=  o.i  Q. 
Current  strength  with  various  combinations  : 
1.05 

1  cell,  ........  I  =  —  —=1.75  amp. 

0.5  +  o.i 

2.        I.O5 

2  cells  in  series,  ....!  =  —  —  1.91  amp. 

2.     0.5  -j-  o.i 

4-      I-°5 

4  cells  in  series,  ....!=-  -  =•  2.0  amp. 

4-     0.5  +  o.i 

Reverse  : 

1.05 

2  cells  in  parallel,  .    .    ,  I  =  —  —  =  3.0  amp. 


1.05 

4  cells  in  series,  ....!  =  —  —  =  4.67  amp. 

0-5 

1+0.1 

4 

After  some  experience  in  electrochemical  work  it 
will  be  easy  to  determine  whether  one  is  confronted 
in  an  experiment  with  a  high  or  low  resistance,  and 
the  manner  of  cell  arrangement  will  accordingly 
follow.  Until  such  experience  has  been  acquired,  the 
safest,  wisest  course  will  be  —  to  experiment  !  Intro- 


2O  ELECTROCHEMICAL    EXPERIMENTS. 

duce  a  measuring  instrument  between  the  battery  and 
the  experimental  cell,  and  vary  the  arrangement  of 
the  cells  until  the  maximum  current  strength  is  ob- 
tained. The  mixed  arrangement  will  be  found  prefer- 
able if  the  cells  employed,  either  in  consequence  of 
their  small  size,  or  from  any  other  causes,  show  a  high 
internal  resistance.  Several  cells  arranged  in  parallel 
will  give  the  same  result  as  one  large  form  of  like  type. 
Meidinger  cells  are  frequently  met  with  in  labora- 
tories. They  are  only  suitable  for  analytical  opera- 
tions in  which  great  current  strength  is  not  required. 
For  electrochemical  experiments  of  any  other  sort 
they  are  worthless,  because  too  many  of  them  must 
be  arranged  in  groups  to  get  currents  of  any  great 
degree  of  intensity. 

STORAGE  CELLS,  ACCUMULATORS,*  SECONDARY 
BATTERIES. 

While  all  primary  batteries  are  more  or  less  incon- 
venient to  handle,  and,  as  a  rule,  furnish  currents  which 
are  not  very  strong,  storage  cells  or  secondary  bat- 
teries are  excellent  sources  of  electric  energy,  and 
serve  for  the  most  varied  electrochemical  experiments. 

Accumulators  consist  of  a  number  of  lead  plates 
which  are  covered  with  a  so-called  "active  mass." 


*A  clear,  concise  description  of  the  action  and  care  of  secondary 
batteries  will  be  found  in  Elbs'  "  Die  Accumulatoren,"  Leipzig,  1893. 
ffoppe,  "  Die  Accumulatoren,"  2te  Aufl.,  Berlin,  gives  a  more  ex- 
haustive and  historical  account  of  their  development. 


SOURCES    OF    THE    CURRENT.  21 

On  the  anode  (positive)  plate  this  is  lead  superoxide, 
while  on  the  kathode  (negative)  plate  it  is  spongy 
lead.  The  liquid  is  pure,  dilute  sulphuric  acid. 
Secondary  batteries  are,  therefore,  galvanic  batteries, 
which,  in  consequence  of  lack  of  diaphragm,  have 
a  vanishingly,  low,  internal  resistance.  When  ex- 
hausted, it  is  not  necessary  to  take  them  apart ;  for 
by  contact  with  a  more  powerful  source  of  energy 
they  can  be  directly  charged — /.  e.,  they  can  be  again 
restored  to  their  normal,  useful  state. 

The  various  modifications  found  in  trade  are  prac- 
tically the  same.  Nearly  all  are  guaranteed  for  a 
longer  or  shorter  period.  The  Tudor  and  Correns  * 
types  are  among  the  best.  There  are  both  stationary 
and  transportable  varieties ;  the  latter  are  closed,  so 
that  no  loss  of  acid  occurs  in  transportation  from  one 
place  to  another. 

The  capacity  of  a  storage  cell  is  given  in  ampere- 
hours.  The  current  strength  varies  with  the  individual 
types,  and  so  long  as  it  is  not  exceeded,  it  is  immaterial 
how  the  electric  energy  is  applied — /.  e.,  whether  a 
powerful  current  is  used  for  a  brief  period  or  a  feeble 
current  for  a  prolonged  period.  A  storage  cell  with 
a  capacity  of  100  ampere-hours  with  a  maximum  cur- 
rent of  10  amp.  may  be  discharged  at  the  rate  of 

10  amperes  for  a  period  of     10  hours,  or 
5         "        "          "       «     20     " 
i         "        "  "        "    100     "        etc. 

*  The  chloride  accumulator  is  most  widely  known  in  this  country. 
— TR. 


22          ELECTROCHEMICAL  EXPERIMENTS. 

Most  factories  give  instructions  as  to  the  manner 
in  which  their  secondary  batteries  shall  be  handled. 
General  rules  may,  however,  be  given  here.  The 
smaller  types  are  usually  mounted.  The  glass  jars 
should  be  thoroughly  cleaned  without  removing  the 
lead  plates.  The  latter  are  then  completely  immersed 
in  pure,  dilute,  cold  sulphuric  acid  of  1.15  sp.  gravity. 
The  purity  of  the  acid  is  important.  The  presence  of 
arsenic  or  nitric  acid  in  it  is  harmful.  If  the  acid  at 
hand  is  not  sufficiently  pure,  conduct  hydrogen  sul- 
phide through  it.  Filter  out  the  precipitated  sul- 
phides, and  expel  the  hydrogen  sulphide  gas  by  means 
of  an  air  current.  When  the  glass  jars  have  been 
filled,  begin  "  charging."  Connect  the  brown  -f-  plates 
with  the  +  conducting  wire  of  a  dynamo  (or  a  ther- 
mopile) and  the  gray  —  plates  with  the  —  pole.  The 
current  recommended  for  the  cell, by  its  manufacturer, 
is  next  conducted  into  it.  As  a  rule,  this  first  charg- 
ing will  extend  through  a  day  or  more.  It  is  in  this 
way  that  the  plates  are  first  brought  into  normal  con- 
dition. The  current  will,  apparently,  be  taken  up 
completely  by  the  accumulator.  The  -f-  plates  grad- 
ually change  in  color  to  a  brownish  black,  while  the 
—  plates  become  a  light  gray.  Eventually,  a  strong 
evolution  of  gas  will  be  observed,  first  on  the  positive 
and  later  on  the  negative  plates  ("the  acid  boils"). 
When  the  evolution  of  gas  on  all  the  plates  remains 
nearly  the  same  for  an  hour,  the  first  "  charge  "  may 


SOURCES    OF    THE    CURRENT.  23 

be  regarded  as  finished.  The  pressure  for  each  cell 
will  have  increased  to  2.5  V. ;  the  specific  gravity  of 
the  acid  will  vary  from  1:15  to  1.18-1.20.  All  of  the 
active  mass  upon  the  positive  plates  will  now  be  com- 
pletely oxidized,  and  that  upon  the  negative  plates 
will  be  fully  reduced.  The  first  discharging  can  next 
follow.  It  should  be  done  with  the  current  strength 
previously  mentioned.  The  pressure  will  fall  rapidly 
to  2.0  V.  and  then  remain  constant  for  a  long  period. 
As  soon  as  it  decreases  to  1.85  V.,  the  discharging 
should  be  interrupted.  The  cells  should  then  be 
charged  a  second  time  until  the  pressure  reaches  2.5 
V.  and  equal  gas  evolution  is  observable  on  all  plates. 
The  cell  is  now  ready  for  further  use. 

To  insure  long  life  to  the  cell,  certain   precautions 
should  be  observed  : 

1.  It  must  be  preserved  from  "  short-circuiting." 

2.  The  maximum  strength  of  the  discharging  cur- 
rent must  not  exceed  that  given  by  the  manufacturer 
of  the  cell. 

3.  Do  not  discharge  below  1.85  V. 

4.  It  should  not  continue  long  in  a  discharged  con- 
dition.    If  not  wanted  for  use,  it  should  be  held  in  a 
charged  state. 

5.  It  is  well  occasionally  to  overcharge — /.  e.,  to  con- 
tinue charging  for  some  hours  with  a  feeble  current 
even  after  the  "  boiling"  has  begun. 

6.  Should  it  become  necessary  at  any  time  to  re- 
move a  plate  from  the  acid,  it  must  not  be  allowed  to 


24         ELECTROCHEMICAL  EXPERIMENTS. 

become  air-dried.  It  should  be  immediately  im- 
mersed in  dilute  acid. 

7.  Avoid  heavy  blows :  they  loosen  and  throw  out 
the  "  active  masses." 

By  observing  these  rules,  storage  cells  can  be  kept 
in  satisfactory  condition  for  some  time,  especially  if 
the  maximum  effect  be  not  constantly  aimed  at.  It  is 
better  to  charge  too  frequently  than  not  enough.  In 
time  the  surface  of  the  liquid  will  sink  below  the 
upper  edge  of  the  plates.  Pure  water  should  then  be 
added.  The  specific  gravity  of  the  acid  should  be 
taken  after  charging  and  discharging.  The  use  of  the 
hydrometer  will  furnish  a  means  of  ascertaining  how 
far  the  charge  has  been  consumed.  If,  in  time,  it  is 
noticed  that,  after  charging,  the  specific  gravity  of  the 
liquid  is  not  the  same  as  was  observed  at  first,  when 
the  cell  was  in  this  condition,  it  is  evidence  that  acid 
has  been  lost  by  spattering  or  in  some  other  way. 
Dilute  acid  should  be  added  several  times  for  the  water 
that  evaporates  until  the  normal  condition  is  restored. 

Storage  cells  answer  well,  both  for  analytical  and 
experimental  purposes.  The  current  and  pressure  of 
any  source  of  electricity  can,  by  this  means,  be 
altered  in  various  ways.  For  example,  a  small 
dynamo  of  5  V.  and  30  amperes,  together  with  six 
accumulators,  each  having  a  maximum  discharge  of 
ten  amperes,  are  at  the  disposal  of  the  operator.  As 

the  machine  is  only  capable  of  charging  ----  =  2  sec- 


SOURCES  OF  THE  CURRENT.  2$ 

ondary  batteries  in  series,  groups  of  three  cells  each 
in  parallel  are  formed,  and  these  then  connected  in 
series  to  the  machine.  When  the  charging  is  finished, 
the  following  groupings  may  be  made : 

I  to  6  in  series  will  yield  2-12  V.  and  10  amp. 
I  "   6  parallel     "       "  2V."    10-60  amp. 

2X3  "       "  4  V.    "    30  amp. 

3X2  «•       «  6  v.     "    20  amp. 

Accumulators  have  found  their  way  into  labora- 
tories slowly  because  dynamos  with  which  to  charge 
them  were  rarely  present.  Recently,  thermopiles  * 
have  been  used  successfully  for  this  work,  hence 
there  now  remains  no  good  reason  for  their  non- 
adoption  generally  in  electrolytic  work. 

THERMOPILES. 

Thermopiles,  which  transform  heat  into  electricity, 
would  be  the  most  convenient  sources  of  electric 
energy  for  laboratory  purposes  if  they  could  be  built 
in  durable  forms.  In  this  respect  the  older  modifica- 
tions of  Noe  and  Clamond  were  sadly  lacking.  They 
required  much  attention  and  a  constant  gas  pressure, 
and,  in  spite  of  all  care,  rarely  lasted  for  any  great 
length  of  time.  The  new  modifications  of  Giilcher  f 
show  decided  improvement  over  the  early  types,  and 
the  common  verdict  in  regard  to  them  is  very  favor- 
able. Julius  Pintsch,  Berlin,  O.,  Germany,  makes 

*  Elbs,  Chem.  Ztg.,  1893,  66.  f  D-  R-   p->  N<>-  44^6- 

c 


26 


ELECTROCHEMICAL    EXPERIMENTS. 


three  varieties  of  this  thermopile  (Fig.  5).  The 
largest  model,  consuming  170  liters  of  gas  per  hour, 
develops  an  electromotive  force  of  4  V.  with  an 
internal  resistance  of  0.6  to  0.7  Q.  The  price  of  this 
form  is  about  $48.  The  smaller  models  consume 
130-70  liters  of  gas,  and  have  a  pressure  of  from  3-1.5 


FIG.  5. 


V.  If  it  be  desired  to  use  the  larger  form  for  charg- 
ing storage  cells,  the  latter  should  be  arranged 
parallel.  This  will  result  in  the  production  of  a  cur- 
rent of  from  2-3  amperes.  For  ordinary  purposes, 
therefore,  a  thermopile  of  this  description  will  be 
ample.  At  present  they  are  used  in  laboratories 


SOURCES    OF    THE    CURRENT.  2? 

chiefly  for  electrolytic  analyses,  but  combined  with 
secondary  batteries  can  be  more  widely  applied. 

DYNAMO-MACHINES. 

The  dynamo  is  the  proper  source  of  electric  energy 
in  all  experiments  requiring  a  powerful  current  for  an 
extended  period. 

The  action  of  such  machines  is  dependent  upon  the 
electric  current  induced  in  a  coil  of  wire  when  brought 
into  a  magnetic  field — i.  e.,  when  it  is  rotated  between 
the  poles  of  a  magnet.  The  portion  that  is  rotated 
is  called  the  armature.  Externally  it  has  the  form  of 
a  flat  ring  or  cylinder.  The  magnets  about  which 
the  armature  rotates  are  not  permanent,  but  are 
electromagnets  excited  by  a  part  of  the  current  pro- 
duced by  the  machine.  The  several  current  impulses 
induced  in  the  armature  are  collected  in  the  commu- 
tator, or  collector,  and  are  given  up  by  the  brushes 
to  the  external  conductors. 

Two  large  classes  of  dynamos  exist:  direct  current 
machines,  and  those  producing  alternating  currents. 
In  the  first  class,  two  successive  current  impulses  have 
the  same  direction,  while  in  the  second  class  they 
proceed  in  opposite  directions.  For  electrochemical 
purposes,  direct  current  machines  are  alone  of  conse- 
quence. 

The  manner  of  winding  also  causes  a  division  into 
series  machines,  shunt  machines,  and  compound 
machines.  In  the  first,  the  current  produced  in  the 


28         ELECTROCHEMICAL  EXPERIMENTS. 

armature  proceeds  immediately  about  the  electro- 
magnets, then  through  the  external  circuit  back  to 
the  armature.  In  the  second,  the  current  divides 
itself  in  the  armature.  The  smaller  portion  of  it 
proceeds  about  the  electromagnets,  while  the  major 
portion  passes  through  the  external  circuit.  In  the 
mixed  or  compound  machines,  a  part  of  the  winding  lies 
in  the  shunt  circuit,  the  remainder  in  the  main  circuit. 

Shunt  machines  alone  interest  us.  They  have  this 
important  advantage,  that  in  consequence  of  any 
disturbances  the  poles  can  not  reverse.  They  will, 
therefore,  be  somewhat  more  closely  considered.  As 
an  example,  or  type,  the  Schuckert  *  flat-ring  machine 
(Fig.  6)  may  be  mentioned. 

The  current  circulating  in  the  external  circuit  is  the 
main  current,  that  about  the  magnets  is  the  shunt  cir- 
cuit of  the  machine.  The  latter  is  interrupted  at  one 
point  for  the  introduction  of  the  shunt-regulator,  N. 
This  serves  to  change  the  pressure  at  the  terminals 
of  the  machine.  If  resistance  be  introduced  by 
means  of  this  regulator  into  the  magnet  winding,  the 
current  passing  through  it  will  be  less  than  before ; 
consequently  the  magnetism  and  the  magnetic  field 
of  the  machine  will  be  reduced  and  the  pressure  will 
fall.  By  suitable  winding  of  the  shunt  regulator  the 
pressure  may  be  varied  within  wide  limits.  Another 
means  of  altering  the  pressure  consists  in  changing 
the  velocity.  When  the  latter  is  increased,  the  pres- 

*  In  this  country  the  Edison  machine  is  generally  used. — TR. 


SOURCES  OF  THE  CURRENT. 


29 


FIG.  6. 


3<D  ELECTROCHEMICAL    EXPERIMENTS. 

sure  increases,  and  vice  versa.  This  course  is  resorted 
to  only  when  extraordinary  conditions  prevail, 
otherwise  the  velocity  allowed  by  the  builder  of  the 
machine  must  be  taken  as  normal.  A  dynamo  with 
an  external  resistance  of  0.068  Q  will  yield 

125  Amp.  —  8.5  V.  —  with  500  revolutions. 
215     "  14.7  V.          "       800  " 

280     "  19.  o  V.         "     1000          " 

Dynamos  are  constructed  for  the  most  varied  cur- 
rent strengths  and  pressures.  In  chemical  operations, 
strong  currents  are  of  prime  importance — the  pres- 
sures are  relatively  low.  Chemical  dynamos  vary,  as 
a  rule,  from  4-20  volts  and  100-500  amperes.  For 
electric  lighting  the  customary  pressures  vary  from 
65-110  V.,  while  the  current  strength  falls  below  100 
amperes. 

The  product  obtained  by  multiplying  together  the 
voltage  and  the  amperage  represents  the  output  of  a 
dynamo.  The  number  of  volt-amperes  is,  therefore, 
a  direct  means  of  measuring  the  mechanical  power  of 
a  dynamo  in  each  moment  of  its  activity. 

A  chemical  dynamo  "  running  empty,"  i.  e.,  when 
it  is  not  connected  with  a  current  circuit,  rapidly  at- 
tains the  pressure  allowed  by  its  prevailing  velocity. 
This  is  called  the  open-circuit  pressure.  It  is  always 
greater  than  that  observed  when  the  machine  is  doing 
work,  or  than  the  pressure  on  closed  circuit.  A  wire 
resistance  connected  with  the  two  terminals  at  once 


SOURCES    OF    THE    CURRENT.  3! 

shows  a  current,  which  maybe  calculated  from  Ohm's 
law.  According  to  the  latter,  the  current  strength 
I  is 

E.  M.  F. 
Sum  of  the  resistances. 

The  electromotive  force  is  the  pressure  of  the  ma- 
chine. The  resistance  is  on  the  outside — the  resist- 
ance W,  while  within  the  machine  there  is  the  resist- 
ance of  the  armature  A  and  the  resistance  of  the  mag- 
net coils  M.  These  two  latter  resistances  are  arranged 
parallel  because  the  dynamo  is  a  shunt  machine.  To- 

A    1VI 

gether  they  equal  -^ — — ;  hence  the  equation  for  the 
current  strength  is 

I  =  ___E_ 


A    +  M 


By  lowering  the  external  resistance,  the  current  in- 
creases and  the  pressure  falls.  The  product  of  the 
pressure  and  current  strength — the  output  of  the 
dynamo — approaches  the  maximum.  By  further  re- 
duction of  the  external  resistance  the  pressure  will 
fall  still  further  ;  the  current  will  increase  for  a  short 
period,  but  will  then  decrease  until,  with  no  external 
resistance  (by  short  circuit  of  the  dynamo),  it  becomes 
zero.  The  machine  then  furnishes  no  current.  The 
voltage  and  amperage  also  rapidly  drop  from  their 
maximum  to  zero. 


32         ELECTROCHEMICAL  EXPERIMENTS. 

This  action  is  related,  too,  with  the  flow  of  current 
toward  the  limbs  of  the  magnet.  So  long  as  the  ex- 
ternal resistance  is  great  in  comparison  with  the 
resistance  of  the  armature  of  the  magnet,  so  long  will 
the  greater  portion  of  the  current  follow  the  more 
convenient  path  about  the  electromagnets.  It  will 
powerfully  excite  these,  and  acquire,  in  consequence,  a 
high  pressure.  As  the  external  resistance  gradually 
grows  less,  the  current  which  flows  about  the  limbs 
of  the  magnet  will  be  diminished,  the  magnetic  field 
of  the  machine  will  become  less  powerful,  also  its 
efficiency,  so  that,  eventually,  when  the  external 
resistance  has,  by  short  circuit,  fallen  to  zero,  a  cur- 
rent will  no  longer  flow  about  the  magnets,  and  the 
machine  will  cease  to  work. 

The  maximum  work  of  a  dynamo  is  attained  when 
the  external  resistance  is  slightly  greater  than  its 
internal  resistance.  This  condition  is  the  basis  for 
the  normal  work  given  by  the  manufacturer  of  the 
machine. 

Smaller  machines,  intended  for  experimental  pur- 
poses,— models, — are  constructed  with  two  separate 
circuits.  These  carry  a  double  coil  upon  the  arma- 
ture. The  main  coil,  with  low  resistance,  gives  up  its 
current  through  the  main  brushes  to  the  external  cir- 
cuit; the  other  coil  has  greater  resistance,  and  the 
current  developed  therein  is  delivered  to  the  magnets 
altogether  in  the  side  brushes.  There  are  also 
machines  with  separate  magnet  excitation.  Should 


SOURCES    OF    THE    CURRENT.  33 

short  circuiting  be  produced  in  such  machines  by 
carelessness,  the  only  resistance  in  the  entire  circuit 
would  be  the  main  armature,  and  all  the  energy  of  the 
machine  would  be  consumed  in  bringing  this  resist- 
ance to  such  a  state  that  it  would  ignite.  In  other 
words,  the  armature  would  be  burnt  out  and  the 
machine  be  ruined.  Direct  current  machines  and 
compound  dynamos  are  damaged  in  the  same  manner. 
It  is  only  the  true  shunt  machine  that  is  non-sensitive. 

The  natural  desire  on  the  part  of  any  one  consider- 
ing the  introduction  of  a  small  dynamo,  is  that  it  shall 
be  suitable  for  every  possible  purpose.  This  wish 
can  not,  of  course,  be  wholly  realized.  It  can  not  be 
expected  that  one  and  the  same  machine  will  serve 
both  for  electrolysis  and  fusion  purposes,  because,  in 
the  first  instance,  low  pressure  and  high  current  are 
required,  while  in  the  second  case  the  requirements 
are  directly  the  reverse — high  pressure  and  moderate 
current.  It  is  always  well  to  have  sufficient  pressure, 
as  it  can  be  easily  reduced,  but  it  is  increased  with 
difficulty.  But  few  electrolytic  operations  require 
more  than  5  V.  pressure,  so  that  this,  in  most  cases, 
would  be  amply  sufficient ;  yet,  to  render  the  machine 
as  widely  useful  as  possible,  it  is  well  to  double  this 
power  for  the  following  reason. 

In  all  technical  operations,  this  question  must  be 
constantly  kept  in  view  :  How  can  this  work  be  done 
on  a  technical  scale  ?  The  work  of  a  dynamo  can, 
however,  only  be  doubled  by  increasing  the  current 


34         ELECTROCHEMICAL  EXPERIMENTS. 

strength  or  by  increasing  its  pressure.  Reasons,  to 
be  given  later,  will  show  that  the  latter  course  is  pre- 
ferable. However,  to  consume  or  exhaust  the  highest 
pressure  the  electrolytic  baths  must  be  arranged 
in  series,  as  in  the  case  of  primary  batteries.  But 
with  this  arrangement,  phenomena  frequently  present 
themselves  which  are  not  noticeable  in  a  single  bath. 
If  the  machine,  then,  has  a  direct  pressure  of  10  V.,  it  is 
even  possible,  with  a  counter  bath  pressure  of  5  V.,  to 
try  series  arrangements.  With  baths  of  lower  pressure, 
it  is,  of  course,  understood  that  there  can  be  more 
baths  arranged  in  series. 

A  machine  having  a  pressure  of  10  V.  and  a  cur- 
rent of  70-100  amperes  will  satisfy  the  greatest  de- 
mands which  can  be  made  upon  a  dynamo  intended 
for  electrolytic  experiments.  The  energy  consump- 
tion will,  therefore,  equal  from  iJ^-2  H.  P. 

RULES  FOR  THE  ARRANGEMENT  AND  RUNNING  OF 
SMALL  DYNAMOS. 

The  machine  should  stand  upon  a  solid  foundation, 
which  can  not  be  easily  shaken.  When  possible,  this 
should  be  a  pier  about  J^  m.  in  height.  It  will  render 
the  observation  of  and  attention  to  the  brushes  much 
easier.  It  would  be  very  convenient  and  practicable 
if  the  dynamo  stood  upon  rails  which  would  permit 
of  its  movement  in  the  direction  of  the  driving  belts. 
Such  a  shifting  is  the  simplest  and  most  convenient 
means  of  stretching  a  belt  that  has  become  loose  and 


SOURCES    OF    THE    CURRENT.  35 

flabby ;  cutting  and  sewing  anew  would  otherwise 
be  necessary.  An  arrangement  permitting  this  can 
be  furnished  for  every  machine  upon  mere  request  to 
the  maker. 

The  transmission  is  best  provided  with  a  triple  cone 
pulley,  so  that  the  middle  step  affords  the  normal 
number  of  revolutions,  and  the  other  two  a  somewhat 
higher  or  a  lower  number.  Avoid  covered  driving 
belts. 

The  bed  of  the  machine  should  be  kept  well  lubri- 
cated. The  "  self-oilers  "  should  always  be  full.  The 
brushes  should  fit  lightly  but  closely.  When  the 
collector  is  pressed  too  closely  it  is  unnecessarily 
abraded  and  heated.  To  insure  long  life  to  the  col- 
lector, it  should  be  constructed  from  hard  bronze, 
while  the  brushes  should  consist  of  soft  copper.  The 
writer's  experience  warrants  the  statement  that  brushes 
from  sheet  metal  are  superior  to  straight  or  twisted 
wire.  They  consist  of  rolled  sheets,  thin  as  paper 
and  soft  as  feathers,  which  slide  along  the  entire  width 
of  the  collector.  In  consequence  of  their  great  elas- 
ticity and  the  absence  of  any  points,  such  brushes  do 
not  spark,  and  the  collector  continues  smooth  for 
quite  a  long  period. 

New  brushes  are  brought  in  contact  with  the  col- 
lector when  the  dynamo  is  running  empty.  Project- 
ing points  produced  by  wear  should  be  cut  off,  because 
they  rapidly  damage  the  machine.  The  brushes  in 
action  require  careful  attention.  When  sparking  sets 


36  ELECTROCHEMICAL    EXPERIMENTS. 

in,  the  binding  screws  should  be  eased  and  the  brushes 
withdrawn  until  the  evil  is  corrected.  In  this  adjust- 
ment care  must  be  taken  not  to  raise  the  brushes  from 
the  collector  during  the  revolution  of  the  latter.  The 
consequence  of  such  an  act  would  be  not  only  the 
production  of  a  blinding  spark,  but,  in  all  probability,  a 
very  severe  shock.  In  disconnecting  the  main  current 
avoid  interrupting  the  conductors  with  both  hands, 
thus  introducing  the  body  into  the  circuit.  This 
always  induces  heavy  shocks.  In  other  respects  the 
various  parts  of  a  dynamo  can  be  handled  during  its 
action  without  experiencing  any  unpleasant  sensation. 

When  the  collector  has  become  rough  and  worn, 
the  brushes  are  removed,  and  it  is  polished  with 
emery  paper.  When  great  inequalities  exist,  a  smooth 
file  may  be  used,  or  the  collector  may  be  made  to 
revolve  around  a  sharp  file. 

The  abrasion  of  the  brushes  and  of  the  collector 
gives  rise  to  a  fine  metallic  dust  which  deposits  on 
all  parts  of  the  machine.  It  must  be  removed  at 
intervals.  This  is  done,  in  the  case  of  portions  more 
difficult  of  access,  by  use  of  a  bellows.  Gentle  tap- 
ping displaces  it  from  the  brushes.  Copper  dust 
should  not  be  allowed  to  accumulate  on  the  collector 
or  its  connections  with  the  armature ;  it  gives  rise  to 
short  circuits  in  individual  segments. 

The  current  is  best  conducted  to  the  baths  by  flat 
metallic  sheets  of  ample  section.  Branches  are  more 
conveniently  made  from  these  than  when  rods  of 


CURRENT    MEASUREMENT.  37 

metal  are  used.  Their  connection  with  the  machine 
is  made  with  strips  of  copper  cord,  so  arranged  that 
the  machine  is  still  movable  on  its  foundation. 


CURRENT  MEASUREMENT. 

Two  courses  maybe  followed  in  measuring  currents. 
The  chemical  action  of  the  electric  current  (volta- 
meter) or  its  magnetic  action  upon  a  conductor 
through  which  it  passes  (galvanometer)  may  be  used 
for  this  purpose.  To  determine  the  current  strength 
in  a  circuit,  i.  e.,  in  an  experiment,  it  is  well  to  insert 
one  of  the  instruments  just  described  into  the  circuit, 
which  consists  of:  battery — experiment — measuring 
instrument — battery.  It  would  be  wholly  wrong  to 
have  the  following  succession  :  battery — instrument — 
battery,  and  then  assume  that  the  current  strength 
thus  observed  was  identical  with  that  from  the  arrange- 
ment :  battery — experiment — battery. 

Electromagnetic  measuring  apparatus  is  calibrated 
in  an  entirely  empirical  way  by  means  of  voltameters, 
therefore  the  latter  will  be  the  first  to  receive  a  brief 
description.  Voltameters  are  based  upon  Faraday's 
law,  which  reads :  "  The  same  current  in  equal  units 
of  time  decomposes  equivalent  quantities  of  chemical 
compounds."  As  representatives  of  the  latter,  we 
may  choose  silver  nitrate,  copper  sulphate,  or  dilute 
sulphuric  acid,  and  then  determine  the  quantities  of 
metallic  copper,  metallic  silver,  or  electrolytic  gas  sep- 


38  ELECTROCHEMICAL    EXPERIMENTS. 

arated  in  a  definite  period  of  time.  Consequently,  we 
distinguish  silver  voltameters,  copper  voltameters,  and 
the  oxyhydrogen  voltameters.  Certain  precautions 
must  be  observed  with  each'to  obtain  accurate  results. 

SILVER    VOLTAMETER. 

This  is  regarded  as  the  most  accurate  voltameter. 
Secondary  reactions  do  not  occur  in  it.  The  high 
equivalent  of  silver,  and  the  fact  that  very  considerable 
quantities  of  metal  are  precipitated  by  comparatively 
feeble  currents,  reduce  the  error  in  weighing  to  a  min- 
imum. A  disadvantage  which  must  be  recognized  is 
that  silver  is  greatly  inclined  to  separate  in  crystals, 
which  are  loosely  attached  to  the  kathode.  Conse- 
quently, stronger  currents  can  not  be  sent  through 
the  apparatus  without  the  possibility  of  some  of  the 
deposit  becoming  detached.  In  measuring  strong 
currents  with  this  instrument,  electrodes  with  a  suffi- 
ciently large  surface  must  be  provided,  besides  which 
the  current  should  not  be  allowed  to  act  for  more 
than  one  to  two  minutes. 

The  voltameter  usually  consists  of  a  silver  or  plati- 
num dish  serving  as  a  kathode,  in  which  there  is  a 
moderately  concentrated  neutral  silver  nitrate  solu- 
tion. The  anode,  of  pure  silver,  is  a  pencil  dipping  into 
the  liquid.  This  anode  is  surrounded  by  a  muslin 
bag  serving  to  collect  any  particles  of  silver  which  may 
separate,  thus  preventing  them  from  falling  upon  the 
dish  and  so  falsely  increasing  the  weight  of  the  latter. 


CURRENT    MEASUREMENT.  39 

A  more  convenient  form  of  this  voltameter  consists 
of  a  small  beaker  glass  containing  the  silver  solution. 
A  sheet  of  pure  silver  serves  as  the  anode,  while  the 
kathode  is  a  plate  of  platinum.  The  liquid  should 
be  agitated  with  a  glass  rod  during  the  action  of  the 
current.  Finally,  the  platinum  plate  is  removed, 
rinsed  with  water,  dried,  and  weighed.  If,  after  sev- 
eral experiments,  the  crystals  have  become  so  large 
that  they  threaten  to  fall  off,  clean  the  platinum 
plate  by  immersion  in  nitric  acid. 

A  current  of  one  ampere  precipitates 

0.001118  gram  Ag  in  one  second,  or 
0.06708       "        "    "     "    minute. 

COPPER    VOLTAMETER. 

In  this  instrument  the  electrodes  are  two  copper 
plates  of  equal  surface.  The  thicker  of  the  two 
serves  as  the  anode,  the  thinner  plate  as  the  kathode. 
They  dip  into  a  solution  of  copper  sulphate.  In  this 
instance,  also,  the  liquid  should  be  thoroughly  agi- 
tated during  the  action  of  the  current.  The  precipitated 
copper  should  have  a  bright  red  color  and  be  perfectly 
adherent.  It  is  first  dipped  into  water,  then  into 
alcohol,  after  which  it  is  dried  over  a  flame.  When 
strong  currents  are  used  anodes  are  placed  on  both 
sides  of  the  kathode. 

Early  data  indicate  that,  as  a  filling  liquid,  an 
almost  saturated  and  perfectly  neutral  copper  sulphate 
solution  answers  best.  It,  however,  increases  the 


4O         ELECTROCHEMICAL  EXPERIMENTS. 

resistance  of  the  voltameter,  in  consequence  of  which 
a  relatively  high  pressure  will  be  required.  Further- 
more, low  results  arise  when  feeble  currents  are  em- 
ployed, because  then  the  copper  which  separates  con- 
tains cuprous  oxide.  It  has  been  shown*  that  per- 
fectly exact  values,  agreeing  with  those  obtained 
by  the  silver  voltameter,  have  resulted  by  using  cur- 
rent densities  varying  from  0.06  to  1.5  ampere  for  a 
sq.  dm.  of  kathode  surface.  This  was  done  by  the 
employment  of  the  following  solution  : 

15  grams  of  crystallized  copper  sulphate. 
5       "       "   concentrated  sulphuric  acid. 
5  c.c.       "  alcohol. 
100   "         "   water. 

The  pressure  (o.  I  —  o.  5V.)  is  only  half  as  large  as 
that  required  for  the  neutral  liquid. 

Such  an  arrangement  of  the  copper  voltameter 
is  well  adapted  for  most  practical  requirements.  Its 
results  are  correct.  It  is  inexpensive  and  easily 
made,  and,  in  addition,  can  be  thrown  into  a  circuit 
for  almost  any  length  of  time.  It  can  also  be  used 
with  advantage  as  an  ampere-hour  meter  (see  p.  5  i). 

One  ampere  of  current  precipitates  0.0197  gram  of 
copper  in  a  minute,  or  1.181  gram  of  copper  per  hour. 

THE  OXYHYDROGEN  VOLTAMETER. 

In  this  voltameter  dilute  sulphuric  acid  (sp.  gr. 
1.1-51.2)  is  decomposed  between  two  platinum 


*  Chemiker  Zeitung,  1893,  543. 


CURRENT    MEASUREMENT.  41 

plates.  The  resulting  electrolytic  gas  is  measured. 
An  ampere  liberates  10.44  c-c-  °f  electrolytic  gas  (at 
o°  and  760  mm.  pressure)  per  minute.  This  kind  of 
voltameter  is  convenient,  because  it  does  away  with 
all  weighings.  The  reduction  of  the  volume  of  gas 


FIG.  7. 

can  be  taken  from  tables,  and  in  a  few  minutes  the 
apparatus  will  again  be  ready  for  a  new  measurement. 
This  convenience,  however,  is  offset  by  the  disad- 
vantage that  the  pressure  required  by  the  voltameter 
is  high.  It  varies  from  1.7  to  2.5  V.  according  to  the 


42         ELECTROCHEMICAL  EXPERIMENTS. 


CURRENT    MEASUREMENT.  43 

size  of  the  instrument.  Therefore,  to  ascertain  the 
current  strength  in  an  experiment  which  demands  a 
pressure  of  4  V.,  it  would  be  necessary  to  employ 
some  source  of  electric  energy  with  a  pressure  of 
about  6  volts ! 

If  an  oxyhydrogen  voltameter  is  to  be  used,  the 
modification  made  by  Kohlrausch  (Fig.  7)  is  preferable. 
The  eudiometer  vessel  is  filled  by  merely  inverting 
the  apparatus.  What  is  known  as  Schmitt's  nitrogen 
eudiometer  (Fig.  8)  can  readily  be  converted  into  a 
practical  electrolytic  voltameter.  The  electrodes  are 
introduced  through  rubber  stoppers.  The  measuring 
burette,  during  use,  is  pushed  somewhat  above  the 
rubber  stopper.  In  reading,  it  is  placed  on  the  latter 
and  brought  to  equal  pressure  by  the  adjustable  level- 
ing tube. 

Voltameters  of  this  description  are  only  in  rare  cases 
adapted  for  current  measurement  by  introduction  into 
the  circuit.  In  order  to  read  them  the.  experiment 
must  generally  be  interrupted,  and  they  augment  the 
resistance  of  the  circuit  to  a  marked  degree.  These 
inconveniences  may  be  avoided  by  the  use  of  measur- 
ing apparatus  based  upon  electromagnetic  principles. 
These  will  now  be  described.  The 

TANGENT    GALVANOMETER 

is  the  simplest  example  of  its  class.  It  consists  of 
a  circular  heavy  copper  wire.  A  magnetic  needle, 
attached  to  an  unspun  silk  fiber,  oscillates  in  this  ring 


44  ELECTROCHEMICAL    EXPERIMENTS. 

over  a  graduated  circle.  The  plane  of  the  ring  is 
placed  in  the  meridian  in  the  direction  of  the  needle. 
The  latter  is  short,  compared  with  the  diameter  of  the 
ring  (not  more  than  one-sixth).  The  magnetic  strength 
of  the  needle  does  not  affect  the  measurement.  When 
the  needle  is  short,  a  more  accurate  reading  can  be  ob- 
tained, even  when  there  is  a  large  divided  circle,  by 
attaching  to  the  needle  a  lighter  and  longer  indicator. 
When  the  needle  is  deflected  «°  out  of  the  meridian, 
the  current  strength  will  be 

1  =  c,  tang.  o. 

Here,  c  indicates  a  constant  value  for  each  instru- 
ment. 

To  determine  this  constant  c  of  the  galvanometer, 
introduce  the  latter,  together  with  one  of  the  volta- 
meters, just  described,  into  the  circuit  of  some  con- 
stant source  of  electric  energy  (the  order  being :  bat- 
tery— voltameter — galvanometer — battery).  Close  the 
circuit  for  a  few  minutes,  note  the  average  deflection, 
and  from  the  reading  of  the  voltameter  calculate 
the  current  in  amperes.  If,  with  a  deflection  of  a°, 
the  current  strength  was  found  to  be  I  amperes,  the 
constant  sought  will  be 

I 

c  •=  ' 

tang,  a 

Several  such  experiments  are  made  with  varying 
currents,  and  from  the  mean  thus  found  the  value  for 
c  is  taken.  A  table  is  then  calculated  once  for  all  to 


CURRENT    MEASUREMENT.  45 

serve  for  the  different  deflections.  To  alter  the  cur- 
rent strength,  an  additional  cell  is  added,  or  a  German 
silver  wire  is  introduced  into  the  circuit  as  resist- 
ance. 

Gaugain  has  made  a  modification  of  the  tangent 
galvanometer  in  which  the  magnetic  needle  does  not 
swing  in  the  center  of  the  ring,  but  is  shifted  from  the 
circular  plane  one-fourth  of  its  diameter.  The  length 
of  the  needle  can  now  be  increased  without  produc- 
ing a  variation  from  the  law  of  tangents. 

Having  constructed  a  table  for  the  galvanometer,  it 
will  soon  be  observed  that  the  measurements  between 
6o°-9O°  are  uncertain,  because  for  each  degree  of 
deflection  the  increase  in  current  will  become  greater. 
It  is  not  well  to  exceed  65°.  In  measuring  larger 
currents,  two  courses  are  open  :  either  a  second  gal- 
vanometer is  used,  which,  with  equal  length  of  needle, 
has  a  larger  ring  diameter,  or,  a  so-called  shunt  is  in- 
troduced. 

When  two  points  in  a  circuit  are  connected,  not  by 
a  single  wire,  but  by  two  wires  of  different  resistances, 
the  current  strength  in  the  two  wires  will  be  inversely 
as  the  resistances.  For  example,  when  the  current  I 
between  the  two  points  A  and  B  (Fig.  9)  meets  the 
two  wires  I  and  II  with  the  resistances  W\  and  W2 
respectively,  it  divides  itself  into  the  two  currents 
ii  and  i2,  between  which  the  following  relations  exist : 

i.  I* +  1  =  1. 

2.  ij  :  i2  =  W2  :  Wr 


46  ELECTROCHEMICAL    EXPERIMENTS. 

If,  for  example,  the  resistance  WT  is  one-third  that  of 
W2,  then  three  times  as  much  current  will  flow 
through  the  former  as  through  the  latter,  or,  when 
referred  to  I,  it  would  be:  ij  ==  ^  I  and  i2  ==  J^  I. 
On  introducing,  therefore,  a  galvanometer  into  a 
circuit  and  connecting  its  terminals  by  means  of  a 
wire,  it  will  be  possible,  through  the  resistance  of 
the  selected  wire,  to  deflect  the  major  portion  of  the 
entire  current  through  this  shunt  and  to  convey  any 
desired  fraction  of  it  through  the  galvanometer.  In 


FIG.  9. 

this  manner  the  capacity  of  the  latter  is  correspond- 
ingly increased.  If  the  shunt  has  a  resistance  equal 
to  that  of  the  galvanometer,  this  would  receive  but 
one-half  of  the  total  current,  hence  its  readings 
should  be  doubled.  If,  in  general,  the  resistance  of 
the  shunt  is  -^  of  that  of  the  galvanometer,  the  read- 
ings must  be  multiplied  by  n  -\-  i.  When,  on  the 
other  hand,  it  is  desired  to  conduct  ~  of  the  total  cur- 
rent through  the  galvanometer,  a  shunt  having  ~-t  of 


CURRENT    MEASUREMENT.  47 

its  resistance  must  be  introduced.  If  the  resistance 
of  the  shunt  is  not  accurately  known,  the  correct 
readings  of  the  galvanometer  can  be  ascertained  by 
a  re-calibration  with  a  voltameter.  The  wire  used  as 
shunt  must  not  be  stretched  out  straight,  otherwise 
the  current  passing  through  it  will  influence  the 
needle.  It  should  be  doubled  and  then  coiled  up. 
In  adjusting  a  galvanometer,  care  must  be  taken  that 
adjacent  currents  do  not  influence  it.  Consequently, 
it  is  connected  to  the  main  circuit  by  two  long  wires, 
lying  closely  together,  so  that  their  inductive  action 
can  be  disregarded. 

By  the  aid  of  a  shunt  it  is  possible  to  employ  very 
sensitive  galvanometers,  having  many  turns  of  wire 
(multipliers  or  galvanometers),  for  the  measurement 
of  current  strength.  The  readings  of  such  an  instru- 
ment can  not  be  deduced  directly  from  a  simple 
formula,  but  the  instrument  itself  must  be  calibrated 
throughout  its  entire  range  with  a  voltameter  (p.  106). 

TORSION    GALVANOMETER. 

The  torsion  galvanometer  of  Siemens  and  Halske 
(Fig.  10)  is  an  exceedingly  delicate  instrument,  and 
for  exact  measurements  is  almost  indispensable.  A 
bell  magnet,  provided  with  copper  damping,  is  sur- 
rounded by  a  coil  of  many  turns,  and  attached  to  a 
spring,  by  whose  rotation  the  deflection  of  the  magnet 
is  compensated  and  restored  to  its  original  position. 

The  magnitude  of  the  angle  of  torsion,  therefore, 


48         ELECTROCHEMICAL  EXPERIMENTS. 

serves  as  a  means  of  measuring  the  current  passing 
through  the  instrument.  The  divisions  are  so  selected 
that  the  wire  coils  have  a  resistance  exactly  equal 
to  I  8,  and  one  degree  of  deflection  corresponds  to 
O.OOI  amp.  With  a  shunt  of  ^  Q  the  reading  must  be 


FIG.  10. 

multiplied  by  10;  i.  e.,  i°  corresponds  to  o.oi  amp.; 
analogously,  with  a  shunt  of 

fo  £2  :      i°  =  o.i  ampere, 

¥lg  12  :     1°  •=  I  ampere. 

As  the  greatest  torsion  angle  equals  170°,  it  is  possi- 
ble, with  such  an  instrument  and  the  three  shunt 
resistances,  which  have  been  mentioned,  to  measure 


CURRENT    MEASUREMENT.  49 

currents   ranging  from    o.ooi-i/o.o  amp.,  which  is, 
obviously,  a  very  wide  range. 

MEASURING    INSTRUMENTS    FOR    TECHNICAL    PURPOSES. 

These  instruments  carry  an  empirical  scale,  which 
gives  direct  readings  in  amperes.  The  instruments 
themselves  can  be  constructed  for  all  current  strengths, 
but  their  range  can  not  be  read  with  the  same 
degree  of  accuracy  throughout.  The  first  divisions 
are  usually  contracted;  hence  their  readings  are  less 
reliable.  Technical  instruments  of  this  class,  called 
amperemeters,  are  indispensable  for  the  control  of 
work  in  a  manufacturing  establishment.  They  give  a 
reading  of  current  strength  in  amperes  at  any  moment. 
They  should  always  be  introduced  into  the  main  cir- 
cuit leading  from  the  source  of  the  current  to  the 
electrolytes.  As  most  amperemeters  are  entirely 
independent  of  the  direction  of  the  current  passing 
through  them,  it  is  immaterial  whether  they  are  con- 
nected with  the  negative  or  positive  conducting  wire. 

Two  phenomena,  as  a  rule,  are  utilized  in  their  con- 
struction :  (i)  A  solenoid,  through  which  the  current 
passes,  tends  to  draw  an  iron  rod  into  its  center  ;  or, 
(2)  a  solenoid  attracts  an  eccentrically  arranged  strip 
of  sheet-iron  toward  the  periphery.  The  two  attrac- 
tive forces  are  opposed,  either  by  gravity  or  by  the 
action  of  a  spring.  The  modifications  of  this  appa- 
ratus are  almost  innumerable.  Two  typical  forms 
will  be  described : 


5O         ELECTROCHEMICAL  EXPERIMENTS. 

The  spring- galvanometer  of  Kohlrausch  (Fig.  11) 
consists  of  a  vertical  solenoid.  In  it  there  is  sus- 
pended a  hollow  sheet-iron  cylinder,  to  which  a  spiral 
spring  is  attached.  When  the  current  passes  through 


FIG.  ii. 


it,  the  iron  cylinder  is  drawn  into  the  solenoid  until 
equilibrium  is  established  by  the  extension  of  the 
spring.  The  latter  then  causes  an  indicator  to  move 


CURRENT    MEASUREMENT.  51 

over  a   vertical   scale.      The    compact   form    of   this 
instrument  recommends  it. 

The  amperemeter  of  Hummel  (Fig.  12),  though  one 
of  the  earliest  constructions,  is  yet  one  of  the  best. 
The  solenoid  in  it  lies  in  a  horizontal  position.  It 
carries  eccentrically  a  thin  sheet  of  iron  also  in  hori- 
zontal position,  bent  and  fixed  at  certain  points.  It 
is  provided  with  a  long  index,  vibrating  before  a 
scale.  The  index  nearly  balances  the  weight  of  the 
iron  strip,  so  that  the  sensibility  of  the  instrument  is 


FIG.  12. 

rather  great.  The  absence  of  magnets  and  springs, 
as  well  as  the  convenient  and  accurate  arrangement 
of  the  apparatus  on  the  zero-point,  when  it  is  mounted, 
contribute  much  to  making  this  a  trustworthy  am- 
peremeter. Its  calibration  is  generally  reliable. 

AMPERE-HOUR    METER.       . 

The   electrolytic    processes,    proceeding    quantita- 
tively according  to   any  single   reaction,  are  few  in 


52         ELECTROCHEMICAL  EXPERIMENTS. 

number.  The  majority  are  accompanied  by  secondary 
changes  which  represent  more  or  less  loss  in  work. 
Hence  a  prime  factor  in  every  electrolytic  process  is 
the  determination  of  the  current  efficiency ;  i.  e.,  the 
ratio  of  the  current  active  in  the  desired  direction  to 
the  total  current  passing  through  the  experimental  cell. 
This  demands  an  arrangement  allowing  the  measure- 
ment of  the  amount  of  current  which  passes  through 
the  circuit  in  a  definite  period,  and  which  is  indepen- 
dent of  the  fact,  whether  the  current  strength  has 
varied  during  the  period  of  observation,  or  whether 
periods  of  cessation  have  actually  occurred. 

This  essential  can  be  realized  in  two  ways  :  By 
use  of  the  electromagnetic  and  the  chemical  action  of 
the  current.  Aron  applies  the  first  means  in  his  reg- 
istering amperemeter.  Two  synchronous  clocks  are 
used  :  one  has  an  ordinary  pendulum,  while  that  of 
the  second  is  controlled  by  an  electromagnet.  Con- 
sequently, during  the  passage  of  the  current,  a  time 
difference  will  be  noticed  in  the  reading  of  the  two 
clocks,  from  which  the  amount  of  current,  which  has 
passed  through  the  instrument,  can  be  calculated. 
This  apparatus  is  extensively  employed  in  measuring 
the  current  consumed  in  electric  light  plants. 

The  amperemeter  of  Kohlrausch  can  be  re-arranged 
so  as  to  serve  for  the  purpose  of  an  ampere-hour 
meter.  This  may  be  accomplished  by  attaching  a 
colored  pencil  to  the  indicator  and  allowing  it  to 
write  upon  a  strip  of  paper  regulated  in  its  move- 


CURRENT    MEASUREMENT.  53 

ments  by  clockwork.  All  the  variations  in  the  cur- 
rent are  recorded  in  this  way.  The  surface  traversed 
by  the  pencil  represents  the  ampere-hours. 

The  copper  voltameter  described  on  page  39  can, 
without  much  expense,  be  changed  into  an  am- 
pere-hour meter.  It  should  be  retained  continuously 
in  the  circuit.  A  glass  or  earthenware  vessel  will 
serve  to  contain  the  electrolyte.  The  anodes  should 
be  copper  plates  varying  from  3  to  10  mm.  in  thick- 
ness. A  thin  copper  plate  will  answer  for  the  ka- 
thode. It  should  be  weighed  before  and  after  the 
experiment  upon  an  ordinary  balance.  One  ampere- 
hour  corresponds  to  1.18  grams  of  copper.  If  the 
electrolyte  is  well  agitated  during  the  experiment,  as 
much  as  2*/2  amperes  can  be  allowed  for  100  sq.  cm. 
of  smooth  kathode  surface.  The  liquid  may  be  agi- 
tated either  by  a  small  stirrer  or  by  conducting  a 
stream  of  hydrogen  gas  through  it.  It  is  not  advis- 
able to  blow  air  through,  because  sulphuric  acid  con- 
taining air  oxidizes  copper,  and  the  results  will,  conse- 
quently, be  inaccurate. 


MEASUREMENT    OF    PRESSURE. 

Ohm's  law  is  thus  expressed  :  E  =  I  .  w.  From 
this  it  is  evident  that  every  pressure  measurement  can 
be  referred  to  a  current  measurement.  To  ascertain 
the  pressure  E,  prevailing  at  the  points  A  and  B  of  a 
circuit,  a  shunt  with  a  resistance  w,  should  be  at- 


54         ELECTROCHEMICAL  EXPERIMENTS. 

tached  to  them,  and  the  current  in  this  branch  be 
then  measured.  The  chief  condition  is  that  the 
quantity  of  current  passing  through  the  shunt  should 
be  an  exceedingly  small  fraction  of  the  main  current, 
hence  w  must  be  very  large  in  proportion  to  the 
resistance  of  the  main  current  between  the  points 
A  and  B.  The  reason  for  this  is  very  evident.  By 
the  attachment  of  the  shunt,  the  resistance  of  the 
conductors  between  the  points  A  and  B  is  reduced, 
because  the  current  has  now  two  outlets.  The  smaller 
the  resistance  of  the  shunt,  the  more  will  the  total  re- 
sistance A  B  be  diminished.  To  produce  the  original 
current  strength  in  the  now  smaller  resistance,  a  lower 
pressure  than  at  first  will  be  required  ;  hence,  by  the 
introduction  of  the  shunt,  the  pressure  is  changed,  and 
is,  indeed,  lowered;  or,  in  other  words,  low  pressures 
are  obtained  with  the  aid  of  a  shunt  of  relatively  less 
resistance.  The  greater  the  resistance  of  the  shunt, 
the  less  will  the  total  resistance  between  A  and  B  be 
altered,  and,  therefore,  the  more  accurately  will  the 
pressure  be  determined. 

The  instrument  for  the  measurement  of  pressure 
— the  voltmeter — is,  therefore,  an  amperemeter  for 
very  low  currents.  It  possess  a  high  resistance  in 
itself,  or  a  great  resistance  is  inserted  before  it.  Its 
divisions,  empirically  deduced,  do  not  correspond  to 
the  current  strengths  which  the  individual  deflections 
produce,  but  they  are  the  product  of  these  current 
strengths  and  the  total  resistance  of  the  instrument; 


CURRENT    MEASUREMENT. 

/'.  e.t  I  .  w,  which  represents  the  pressure  according 
to  Ohm's  law,  expressed  in  volts. 

Every  sensitive  galvanometer  may  be  calibrated  to 
do  the  work  of  a  voltmeter  by  the  insertion  of  a 
sufficiently  large  resistance  (p.  1 1 1).  The  capacity  of 
every  voltmeter  can  be  doubled  by  the  insertion  of  a 
resistance  equal  to  that  of  the  instrument  itself;  and 
when  the  resistance  is  twice  as  great,  the  capacity  is 
trebled,  etc.,  etc. 

The  methods  of  pressure  measurement,  which  are 
determined  by  the  comparison  of  an  unknown  electro- 
motive force  with  that  of  a  normal  element,  can  be 
omitted  here,  as  they  are  all  rather  inconvenient. 
The  chemist,  engaged  in  electrochemical  work,  must 
be  able  to  know,  in  a  very  few  minutes,  both  the 
pressure  and  current  strength,  either  by  deflection  of 
a  needle  or  by  some  other  indicator.  It  is  only  by 
this  means  that  he  can  rapidly  attain  his  purpose,  and 
have  full  oversight  of  his  experiments  at  any  moment. 

The  torsion  galvanometer  is  not  only  an  excellent 
aid  in  determining  current  strength,  but  it  can  also 
be  applied  in  measuring  pressure.  This  may  be 
accomplished  by  inserting  resistances  of  varying 
degree.  This  galvanometer  has  an  internal  resistance 
of  one  ohm,  and  i°  of  deflection  represents  o.ooi 
ampere.  When  pressure  is  to  be  measured,  i°= 
o.ooi  X  i  —  o.ooi  V.  By  the  insertion  of  9  Qt  the 
total  resistance  of  the  apparatus  becomes  10  Q\  that 
is,  it  is  ten  times  greater  than  at  first,  and,  in  conse- 


56         ELECTROCHEMICAL  EXPERIMENTS. 

quence,  1°  deflection  corresponds  to  ten  times  its 
original  value,  which  also  follows  from  the  formula 
of  Ohm  :  E  =  o.ooi  X  10  —  o.oi  V. 

The  insertion  of  99  Q  raises  the  total  resistance  to 
100  @,  and  1°  deflection  corresponds  to  o.ooi  X  100 
=  0.1  V. 

The  insertion  of  999  Q  increases  the  entire  resist- 
ance of  the  apparatus  to  1000  @,  and  1°  deflection 
corresponds  to  o.ooi  X  1000  ==  I  V. 

For  convenience,  the  three  resistances,  9,  99,  and 
999  $,  are  introduced  into  a  box,  and  can  be -inserted 
in  the  circuit  by  means  of  a  plug.  To  avoid  injuring 
the  apparatus,  when  ascertaining  unknown  pressures, 
it  is  best  to  throw  in  the  greatest  resistance  at  first, 
then  proceed  downward  to  the  lowest.  The  poles 
should  be  so  arranged  that  the  magnetic  needle  is 
deflected  in  a  direction  opposite  to  the  graduation. 
By  turning  the  torsion  screw  in  the  line  of  graduation, 
the  indicator  is  again  brought  back  to  the  zero  mark, 
and  the  angle  of  torsion  may  then  be  read.  If  this 
be  too  small,  the  torsion  spring  is  released  by  turn- 
ing it  back  to  the  zero  mark,  and  a  plug  is  inserted  at 
the  next  highest  point  of  resistance.  This  is  repeated 
until  the  angle  of  deflection  is  sufficiently  large. 

The  principles  of  construction  in  all  amperemeters 
now  in  use  permit  of  the  application  of  the  latter  as 
voltmeters,  hence  these  occur  in  the  greatest  varieties 
and  with  the  greatest  differences  in  capacity.  Ex- 
ternally, they  resemble  the  amperemeters,  but  differ 


CURRENT    MEASUREMENT.  57 

from  them  in  having  many  more  coils  of  wire  and 
in  their  high  resistance. 

Amperemeters  and  voltmeters  containing  spirals  or 
springs  should  be  tested,  from  time  to  time,  as  to  their 
accuracy,  particularly  if  they  are  to  be  used  con- 
tinuously in  a  circuit. 

REGULATION    OF    CURRENT. 

The  previous  chapters  have  dealt  with  the  produc- 
tion of  the  current  and  its  measurement,  both  as  to 
intensity  and  pressure.  The  methods  by  which  it 
can  be  altered  within  any  desired  limits  will  be  next 

considered. 

•p 

It  follows  from  Ohm's  law:     I  =    — ,    that    I    can 

lw 

be  altered  in  two  ways :  either  by  alteration  of  pres- 
sure, the  total  resistance  remaining  the  same,  or  by 
changing  the  total  resistance,  the  pressure  remaining 
the  same. 

A.     ALTERATION      OF     CURRENT     STRENGTH     BY     CHANGE 
IN    PRESSURE. 

With  primary  or  secondary  batteries,  the  pressure 
may  be  varied  by  arranging  a  smaller  or  larger  num- 
ber in  series.  It  has  already  been  stated  that,  in  the 
case  of  batteries  with  high  resistance,  the  introduc- 
tion of  each  cell  into  the  circuit  is  accompanied  by 
an  appreciable  change  in  the  expression  Jw. 

With  dynamos  it  is  possible  to  alter  the  pressure, 


58         ELECTROCHEMICAL  EXPERIMENTS. 

without  disturbing  the  construction  of  the  machine, 
by  changing  the  velocity  or  by  altering  the  resistance 
in  the  magnet  coils.  An  example  was  given  on  page 
30,  showing  how,  by  increasing  the  velocity,  the  pres- 
sure on  open  circuit  was  increased,  and  mention  was 
also  made  that  in  setting  up  a  dynamo  for  experi- 
mental purposes  it  is  well  to  provide  it  with  a  cone- 
pulley,  so  that  the  middle  pulley  would  cause  the 
normal  velocity  and  the  other  two  either  a  higher  or 
a  lower  speed.  If  the  variations,  above  or  below, 
are  maintained  within  20  per  cent,  of  the  maximum 
or  minimum,  there  is  no  danger  of  doing  harm  to 
the  machine.  This  is  furthermore  true,  because,  in  a 
machine  used  in  an  experimental  plant,  the  require- 
ments are  not  continuous,  but  only  temporary,  and 
for  conditions  that  are  extraordinary. 

The  second  method  of  altering  the  pressure  con- 
sists in  inserting  a  resistance,  by  means  of  the  regu- 
lating shunt,  in  the  limb  of  the  magnet  (p.  28).  This, 
however,  merely  lowers  the  pressure.  If  the  regulator 
supplied  by  the  manufacturer  allows  only  a  moderate 
reduction,  it  can  be  connected  with  larger  resistances. 

B.     ALTERATION     OF    CURRENT     STRENGTH     BY     ALTERA- 
TION   IN    TOTAL    RESISTANCE. 

Two  distinct  conditions  may  occur  here.  Increase 
in  current  strength  may  be  achieved — 

I.  By  reduction  of  the  internal  resistance  of  the 
source  of  the  current.  This  refers  especially  to  bat- 


CURRENT    MEASUREMENT.  59 

teries.  Their  internal  resistance  may  be  diminished 
by  placing  several  of  them  parallel,  or  by  the  use  of 
a  larger  form  of  the  same  kind  of  battery.  This  is 
also  true  of  storage  cells,  but  it  must  be  observed 
that  such  conditions  rarely  obtain,  because  this  source 
of  current  has  but  slight  internal  resistance.  It  can 
only  happen  should  the  current  required  for  an 
experiment  be  greater  than  the  heaviest  discharge 
allowed  for  this  type  of  battery. 

The  internal  resistance  of  dynamos  can  not  be 
diminished. 

2.  By  reduction  of  the  external  resistance.  Under 
this  head  the  conducting  wires  must  be  considered. 
Those  used  in  electrolytic  experiments  should  be  as 
short  as  possible  and  not  too  light  in  weight.  For 
the  minor  experiments,  good  copper  wires  of  2  mm. 
diameter  will  be  sufficient.  It  is  customary  to  allow 
about  from  2  to  3  amperes  for  a  copper  wire  with  a 
cross-section  of  one  sq.  mm. 

The  resistance  of  the  experimental  cell,  or  "  bath," 
as  it  is  usually  termed,  may  be  reduced — 

(1)  by  increasing  the  electrode  surfaces, 

(2)  by  diminishing  their  distance  from  one  another, 

(3)  by  increasing  the  conductivity  of  the  electrolyte, 

(4)  by  using  a  thinner  or  more  porous  diaphragm, 
if  such  is  required  by  the  bath. 

It  often  happens,  in  using  some  source  of  electricity, 
— e.g,  a  dynamo  or  an  aggregation  of  storage  cells, 


60  ELECTROCHEMICAL    EXPERIMENTS. 

where  there  is  abundant  pressure  and  but  slight  inter- 
nal resistance, — that  the  current  and  pressure  must  be 
reduced  to  some  definite  value.  This  can  be  accom- 
plished by  throwing  resistances  into  the  circuit. 
German  silver  wires  (or  nickelin  or  rheotan),  of  vary- 
ing diameters,  arranged  in  spiral  or  zigzag  form  upon 
a  frame  of  wood,  answer  well  for  this  purpose.  At 
suitable  distances  solder  on  short,  thick  copper  wires, 
which  dip  into  mercury  cups.  When  such  a  resist- 
ance is  introduced  into  the  circuit,  the  current  is  com- 
pelled to  pass  through  all  the  wire  divisions ;  and  in 
order  that  the  resistance  may  be  lowered,  it  is  usual  to 
throw  out  one  or  several  of  the  divisions.  This  is 
generally  done  by  connecting  the  corresponding  mer- 
cury cups  with  a  copper  wire  having  this  shape  |— 1. 
To  screw  the  individual  wires  to  metal  blocks,  and 
throw  out  the  several  divisions  by  introducing  plugs, 
is  less  worthy  of  recommendation,  because  usually 
the  metallic  blocks  soon  become  worn  and  the  plugs 
do  not  fit  well.  Furthermore,  such  plug  contacts  do 
not  long  remain  bright  and  clean  in  a  laboratory  at- 
mosphere. The  adjustable  rheostats  (Fig.  13)  are 
better.  In  them  the  individual  resistance  divisions 
are  directed  to  metal  buttons  ;  over  these  passes  the 
movable  arm. 

The  resistances  of  the  several  divisions  of  wire  are 
numbered  as  is  done  in  the  case  of  a  box  of  weights  : 
I,  2,  2,  5,  IO,  20,  20,  50,  etc.,  U ;  or,  by  doubling  each 


CURRENT    MEASUREMENT. 


6l 


value,  i,  2,  4,  8,  16, — Q.  By  the  latter  plan  more  is 
accomplished  with  the  same  number  of  resistances, 
but  the  arrangement  is  less  convenient. 

The  resistance  of  a  wire  is  greater,  the  thinner  it  is  ; 
hence,  it  might  be  thought  that  it  would  be  more 
advantageous  to  use  a  very  light  wire,  for  then  the 


FIG.  13. 

minimum  quantity  of  wire  would  be  consumed.  This 
conclusion  is,  however,  false.  The  electric  energy 
annihilated  by  a  wire  is  transformed  into  heat.  On 
sending  a  current  of  moderate  strength  through  a 
very  thin  wire,  the  increase  in  temperature  could  rise 
to  the  ignition  point,  or  even  to  the  point  effusion  of 


62         ELECTROCHEMICAL  EXPERIMENTS. 

the  wire.  In  making  calculation  for  a  current  regu- 
lator, the  intensity  of  the  current  passing  through 
each  division  or  section  must  be  considered.  The 
following  table  gives  results  obtained  in  experiment- 
ing with  nickelin  wires;  it  may  prove  of  assistance  in 
making  the  calculation  for  current  regulators  : 


DIAMETER  IN 

RESISTANCE  FOR 

IGNITION  BEGINS 

MM. 

i  M.  LENGTH  IN  Q. 

WITH  AMPERES. 

O.2 

13 

i-7 

0.4 

3-2 

4 

0.6 

1.41 

7 

0.8 

0.79 

9l/2 

I.O 

0.51 

14/2 

1-25 

0.33 

20 

1.50 

0.23 

32 

i-75 

0.16 

40 

2.O 

0.13 

45 

The  current  strength  given  here  should  not  be 
actually  reached,  because  ignition  always  proves 
destructive  to  resistance  wires  (p.  1 16).  With  power- 
ful currents,  such  as  dynamos  give,  wires  will  no 
longer  answer,  because  the  cooling  of  a  wire  by  in- 
creasing its  thickness  is  always  unfavorable.  Hence, 
it  is  preferable  and  better  to  take  short  strips  of  thin 
sheet  nickelin.  These,  with  a  section  equal  to  that 
of  a  wire,  can  carry  much  heavier  currents  and  yet 


not  ignite. 


THE    VESSELS.  63 

STRIPS    OF   NlCKELIN,  0.3    MM.    IN    THICKNESS. 


WIDTH  IN  MM.  RESISTANCE  FOR  MAXIMUM  CHARGE 

i  M.  LENGTH  IN  U.  IN  AMPERES. 


IO 

0.133 

40 

15 

0.0889 

60 

20 

0.0667 

80 

25 

0-0533 

95 

30 

0.0444 

no 

35 

0.0381 

130 

40 

0.0333 

145 

45 

0.0296 

160 

5o 

0.0267 

175 

The  maximum  charges  given  above  are  so  measured 
that  the  sheets  of  metal,  under  the  ordinary  methods 
of  cooling,  never  reach  the  ignition  point.  They  will 
not  fuse  through  until  the  current  is  doubled  or 
trebled.  Several  metal  sheets  can  be  screwed  or  sol- 
dered together. 


B.  ARRANGEMENT  OF  EXPERIMENTS. 

THE  VESSELS. 

In  the  initial  experiments,  when  electrolysis  is  to 
be  employed,  it  is  best,  where  possible,  to  use  glass 
vessels,  simply  because  the  latter  permit  the  eye  to 
observe  many  of  the  changes  occurring.  For  exam- 
ple, by  transmitted  light,  alterations  in  concentration 
at  the  electrodes  may  be  detected  by  the  formation 


04         ELECTROCHEMICAL  EXPERIMENTS. 

of  currents  in  the  liquid;  alterations  in  color,  the  in- 
tensity of  the  gas  evolution  at  different  points  on  the 
electrodes,  the  separate  stages  in  the  production  of 
precipitates  at  the  kathode,  and  the  solution  of  the 
anode,  as  well  as  other  points  of  interest,  can  be  noted. 
The  general  introduction  of  storage  cells  of  many 
sizes  has  made  it  possible  to  obtain  glass  jars  of  the 


FIG.  14. 

most  varying  dimensions.  Smaller  vessels  may  be 
prepared  by  painting  cigar-boxes,  etc.,  on  the  interior 
and  exterior  with  paraffin.  Such  baths  answer  for 
both  acid  and  alkaline  liquids.  They  will  last  for 
several  weeks  at  least. 

With  experiments  conducted  on  a  larger  scale, 
earthenware  troughs  or  boxes,  such  as  are  now 
used  in  galvanizing  establishments,  are  much  em- 
ployed. They  are,  however,  being  gradually  sup- 


DIAPHRAGMS.  65 

planted  by  wooden  vessels  lined  with  lead  (Fig.  14). 
The  latter  are  made  of  stout  planks,  provided  with 
tongue  and  groove,  with  addition  of  a  tarred  string, 
and  are  tightly  bound  together  and  screwed  up  with 
bolts.  The  inner  surface  of  these  receptacles  is  cov- 
ered with  tar  or  pitch,  or,  what  is  better,  with  sheet- 
lead.  The  separate  lead  sheets  are  then  welded 
together  at  their  edges  with  an  oxyhydrogen  flame, 
so  that  the  receptacle  is  really  a  water-tight  lead  box, 
resting  in  a  frame  of  wood. 


DIAPHRAGMS. 

When,  in  an  electrolytic  process,  substances  are 
produced  at  one  or  at  both  electrodes,  which  prove 
to  be  soluble  in  water,  it  is  impossible,  unless  suitable 
means  are  adopted,  to  prevent  the  separated  material 
from  mixing.  Substances  thus  separating  and  again 
mixing  destroy  one  another,  or  react  upon  one  another 
in  a  very  undesirable  way.  To  obviate  this,  a  porous 
diaphragm  is  introduced  between  the  electrodes.  It 
should,  first  of  all,  prevent  the  products,  appearing  at 
the  electrodes,  from  mixing.  Next,  it  should  offer 
little  resistance  to  the  passage  of  the  current,  and  it 
should,  from  a  mechanical  standpoint,  be  firm  and 
durable,  and  not  affected  either  by  the  chemicals 
present  in  the  bath,  or  by  any  which  may  be  pro- 
duced in  it.  The  material  composing  it  should  be 
easily  obtained  and  inexpensive.  These  requirements 


66         ELECTROCHEMICAL  EXPERIMENTS. 

are  fulfilled  with  difficulty,  hence  anything  approxi- 
mating the  chief  requirements  must  suffice.  The 
preparation  of  a  diaphragm,  satisfactory  in  every  par- 
ticular, is  still  a  problem  of  the  future.  Wide  fields 
will  be  opened  up  to  electrolysis  when  once  this  de- 
sideratum is  found.  The  discovery  of  a  diaphragm 
suitable  for  acid  and  also  for  alkaline  liquids,  would 
certainly  bring  a  high  pecuniary  reward.  Recently, 
efforts  have  been  made  to  conduct,  on  a  technical 
scale,  operations  requiring  diaphragms,  but  without 
success,  and  the  consequence  has  been  that  manufact- 
urers have  turned  again  to  those  processes  and  ma- 
terials in  which  these  disturbing  factors  (diaphragms) 
are  not  present. 

Bunsen,  in  preparing  metallic  magnesium  from  the 
fused  chloride,  brought  into  notice  the  simplest  form 
of  diaphragm.  He  introduced  a  partition  from  above 
into  the  fused  mass ;  two  communicating  chambers 
were  thus  formed,  and  into  them  he  introduced  the 
electrodes.  Chlorine  can  not  remain  dissolved  in  the 
fused  mass;  it  rises  immediately  to  the  surface,  where 
it  distributes  itself  and  escapes  with  much  foaming. 
The  diaphragm  prevents  it  from  passing  into  the 
second  chamber,  where  the  magnesium  collects  in 
little  globules. 

In  galvanic  batteries  we  noticed  that  the  porous  cup 
acted  really  as  a  diaphragm.  These  cups  answer  well 
or  poorly  for  the  work  for  which  they  are  constructed, 
depending  upon  the  material  and  the  temperature  at 


DIAPHRAGMS.  67 

which  they  have  been  baked.  A  method  for  testing 
them  has  already  been  given  on  p.  13,  and  it  is  not 
necessary  to  repeat  the  same  here.  For  acid  liquids 
porous  cups  answer  well  enough,  but  they  are  useless 
with  alkaline  liquids,  because  the  latter  decompose 
and  soon  destroy  them.  When  experimenting  on  a 
small  scale,  the  porous  cups  are  satisfactory  enough  as 
diaphragms,  but  in  technical  operations  their  compara- 
tively high  price,  and  their  short  life,  due  ofttimes  to 
their  fragility,  prevent  their  general  adoption.  When 
the  products  sought  are  costly,  the  use  of  diaphragms 
is  to  be  recommended.  The  cylindrical  form  of  por- 
ous cup  is  usually  quite  inconvenient  in  the  construc- 
tion of  apparatus,  hence  it  will  be  found  to  be  more 
practical  to  use  plates  of  porous  clay  ;  these  should 
be  placed  in  the  baths  as  diaphragms.  The  formation 
of  such  chambers  as  would  result  in  this  way  must 
produce  no  insurmountable  difficulties. 

Quite  frequently  the  sole  purpose  of  the  diaphragm 
is  to  prevent  pulverulent  substances  separated  at  one 
electrode  from  passing  to  the  other  electrode.  In  such 
instances  the  diaphragm  is  really  nothing  more  than 
a  filter,  so  that  a  bag  of  silk  or  muslin  can  be  attached 
to  the  respective  electrode  (see  old  form  of  silver  vol- 
tameter, p.  38). 

The  material  used  in  technical  operations,  at  least 
in  an  experimental  way,  for  diaphragm  purposes,  has 
been  parchment  paper,  felt,  or  asbestos. 


68  ELECTROCHEMICAL    EXPERIMENTS. 


THE  ELECTRODES. 

To  save  space,  electrodes  are  generally  in  plate  or 
sheet  form.  These  are  hung  parallel  in  the  bath. 
This  arrangement  allows  the  use  of  both  sides  of  the 
electrodes.  When  the  purpose  is  to  throw  a  metal 
out  of  solution,  a  sheet  kathode  of  the  same  metal 
should  be  used  to  get  rid  of  impurities.  Otherwise 
the  choice  of  kathode  material  is  merely  dependent 
upon  the  nature  of  the  liquid  which  it  is  proposed  to 
decompose.  A  favorable  circumstance  which  maybe 
mentioned  in  this  connection  is,  that  the  kathode  is 
protected  from  corrosion  under  the  liquid,  by  the  re- 
ducing action  of  the  current.  '  In  acid  liquids  lead  or 
copper  is  employed,  while  iron  is  used  in  alkaline 
liquids.  The  electrodes  should  dip  far  into  the  liquid, 
first,  in  order  that  the  material  may  be  used,  second, 
because  the  salts  of  the  solution  creep  up  and  strongly 
corrode  that  portion  of  the  electrode  which  is  not  im- 
mersed. Particular  attention  should  be  directed  to 
the  points  of  contact,  otherwise  great  resistance  may 
be  introduced,  and,  indeed,  the  current  can  eventually 
be  entirely  interrupted.  Exposed  points  should  be 
protected  by  a  layer  of  varnish. 

Anodes,  according  to  their  deportment  toward  a 
current,  are  divided  into  two  groups.  The  first  group 
comprises  those  anodes  which  decompose  and  are  dis- 
solved— the  soluble  anodes.  In  the  second  group  are 
the  insoluble  anodes.  Soluble  anodes  should  be  used 


THE    ELECTRODES.  69 

if  it  can  possibly  be  done  in  the  process  under  study. 
The  insoluble  anodes  are  platinum,  lead,  and  carbon 
for  acid  and  neutral  solutions,  while  iron  is  used  in  al- 
kaline liquids. 

At  the  ordinary  temperatures  platinum  anodes  are 
not  attacked  in  solutions  of  any  kind,  not  even  by  the 
halogens ;  hence  they  are  the  most  satisfactory  ma- 
terial for  experimental  purposes.  Their  high  price 
prevents  their  general  adoption  in  technical  work. 
The  attempt  has  been  made  to  substitute  platinum 
coated  sheets  for  plates  of  the  pure  metal,  without, 
however,  any  degree  of  success.  The  platinum  coated 
material  is  not  sufficiently  dense  and  adherent  to  pre- 
vent the  solution  of  the  metal  beneath  it.  With 
platinum  anodes,  the  union  between  the  plate  and  the 
wire  is  always  effected  by  rivets  or  by  welding,  but  not 
by  soldering,  as  the  gold  used  in  soldering  is  not  only 
dissolved  by  the  halogens,  but  also  by  the  oxygen 
acids  during  the  electrolytic  process. 

Sheets  of  lead  form  very  satisfactory  anodes  in 
solutions  of  pure  sulphuric  acid.  They  soon  become 
coated  with  a  brownish-black,  protecting  layer  of 
lead  dioxide  (PbO2),  otherwise  oxygen  appears  upon 
them  just  as  it  does  with  platinum.  (The  author  has 
observed  that  the  oxygen  liberated  upon  a  lead  anode 
is  free  from  ozone.)  The  lead  dioxide  slowly  sepa- 
rates in  leafy  scales,  which  are  increased  by  interrup- 
tion of  the  current.  The  consumption  of  lead,  how- 
ever, is  in  no  sense  more  appreciable.  In  the  presence 


70         ELECTROCHEMICAL  EXPERIMENTS. 

of  halogens  and  nitric  acid,  the  lead  is  attacked  more 
rapidly,  but  irregularly,  with  the  production  of  white 
layers  of  chloride  or  sulphate  (p.  87). 

When  there  is  an  evolution  of  chlorine,  the  third 
substance,  carbon,  is  stable,  but  it  is  strongly  attacked 
in  all  cases  where  oxygen  is  evolved.  Carbon  anodes 
serve  admirably  in  hydrochloric  acid  solutions,  but  in 
sulphuric  acid  they  are  visibly  affected,  large  black 
scales  falling  off,  while  a  brown  dye-stuff  dissolves. 
The  resistance  of  carbon  being  so  much  greater  than 
that  of  the  metals,  the  carbon  anodes  must,  conse- 
quently, be  of  much  larger  cross-section.  The  con- 
ductivity and  the  power  of  resisting  the  action  of 
chemicals  are  greater,  the  harder  and  denser  the  car- 
bon is;  therefore,  cut  retort-carbon  is  better  than  the 
artificial  and  compressed  carbon.  The  creeping  of 
solutions  on  the  porous  carbon  plates  is  disagreeable 
and  troublesome,  causing  destruction  of  the  contact 
points.  It  may  be  overcome  by  paraffining  the  carbons 

(p.  12). 

Thus  far  no  material  has  been  discovered  which 
will  answer  as  a  durable  anode  in  electrolytic  pro- 
cesses where  mixtures  of  chlorine  and  oxygen  are 
produced.  Platinum  is  too  expensive  and  carbon  is 
consumed  too  rapidly.  A  process  of  this  class — a 
process  of  eminent  importance — in  which  this  diffi- 
culty is  keenly  felt,  is  the  electrolysis  of  sodium 
chloride  for  the  manufacture  of  chlorine  and  caustic 
soda. 


THE   CONDUCTORS.  /I 

THE  CONDUCTORS. 

Soft  copper  is  invariably  used  to  conduct  the  cur- 
rent. In  minor  experiments,  the  ordinary,  refined  cop- 
per with  a  conductivity  of  from  52  to  56  will  suffice. 
In  technical  operations,  electrolytic  copper  is  used. 
Its  conductivity  varies  from  58.5  to  60.  If,  at  any  time, 
it  is  necessary  to  throw  in  a  flexible  piece,  experi- 
ments on  a  small  scale  will  require  only  a  thin  spiral, 
but  in  the  larger  operations  a  piece  of  copper  cable  will 
be  the  most  suitable.  The  contacts  must  all  be  kept 
bright.  When  they  become  dull,  as  happens  at  the 
electrodes,  it  should  be  the  care  of  the  operator  to 
inspect  them  frequently  and  brighten  them  up.  The 
cross-section  is  so  chosen  that  in  conductors  of  mod- 
erate length,  say  up  to  20  meters,  not  more  than  3 
amperes  will  fall  to  I  sq.  mm.  It  follows  from  this 
that  in  using  powerful  currents  large  cross-sections 
are  requisite,  and  consequently  that  conducting  rods 
of  very  considerable  cost  are  necessary.  It  is  best, 
in  general,  not  to  use  currents  exceeding  500  amperes. 
To  increase  the  production,  and  yet  use  a  single 
machine,  it  is  better  to  increase  the  pressure  than  to 
increase  the  current  strength.  To  conduct  or  dis- 
tribute the  current  from  the  main  line,  screw  clamps 
of  the  most  varied  construction  may  be  used  (Fig. 

is). 

The  screw  a  is  used  to  connect  two  wires  ;  it  is 
pushed  over  the  one  wire,  b  and  c  are  very  conveni- 


72         ELECTROCHEMICAL  EXPERIMENTS. 

ent  modifications  for  the  construction  of  apparatus. 
They  are  either  screwed  directly  into  wood  (b),  or, 
like  the  table  connector  c,  are  fastened  to  a  table  by 
means  of  two  screws,  d  serves  to  connect  a  sheet 
and  wire  conductor,  e  is  used  to  connect  larger  con- 
ducting rods.  The  hinge  arrangement  permits  of  its 
attachment  from  without. 

Cylindrical  rods  are  not  well  suited  for  conducting 
purposes,  because  the  borings  in  the  binding  screws 


FIG.  15. 

are  only  intended  for  a  single  wire.  A  much  more 
convenient  arrangement  is  the  flat  conducting  wire. 
This  can  then  be  attached  to  the  machine,  and  there 
being  a  larger  contact  surface,  one  can  be  assured  of 
a  more  complete  distribution  of  the  current.  The 
"  rider "  binding  screw  /  can  be  clamped  to  this. 
The  binding  screw  g  facilitates  conduction  by  means 
of  the  round  wire. 


THE    ELECTROLYTE.  73 

An  essential  for  every  electric  conductor  is  the 
"  key  "  or  "  switch."  At  the  completion  of  every  ex- 
periment, the  main  current  must  be  interrupted.  This 
is  necessary  in  order  to  prevent  a  reverse  discharge 
of  the  experimental  cell.  In  minor  experiments  the 
wires  are  merely  withdrawn  from  the  binding  posts  or 
screws.  In  larger  or  technical  operations  an  adjust- 
able rheostat  is  used.  The  contact  of  the  latter 
breaks  the  entire  current.  To  renew  the  electrolysis, 
the  dynamo  is  permitted  to  run  until  it  has  gained 
its  maximum  speed  and  pressure ;  then  the  crank  or 
arm  of  the  switch  is  slowly  moved  over  the  individual 
resistances  until  the  point  is  reached  at  which  full 
current  again  sets  in.  The  current  and  energy  con- 
sumption are  gradually  raised  to  the  maximum;  a 
damaging  loading  of  the  machine,  by  jerks,  is  thus 
avoided ;  for  this  would  happen  were  the  full  current 
turned  on  at  once. 


THE  ELECTROLYTE. 

Not  much  in  general  is  to  be  said  concerning  the 
electrolyte.  It  may  be  either  crude  material  to  be 
changed  by  the  electrolysis,  and  then  its  properties 
are  regarded  as  assumed  in  the  purchase ;  or,  it  is  a 
means  to  an  end — e.g.,  the  purification  or  refinement  of 
metals.  Then  the  effort  is  made  to  have  its  chemical 
properties  such  that  they  will  answer  for  the  require- 
ments of  the  designed  process.  In  the  refinement  of 


74  ELECTROCHEMICAL    EXPERIMENTS. 

copper,  the  electrolyte  is  the  sulphate,  because  lead, 
antimony,  tin,  and  gold  dissolve  in  it  with  difficulty.  If 
a  chloride  solution  should  be  employed,  the  metals 
mentioned  would  first  pass  into  solution,  and  later  they 
would  reappear  in  the  precipitate. 

The  electrolyte  should  be  as  good  a  conductor  as 
possible,  so  that  little  energy  will  be  required  to 
overcome  its  resistance.  Conductivity  increases 
with  increasing  concentration,  by  acidulation,  and 
by  the  application  of  heat.  A  maximum  conduc- 
tivity is  imparted  to  many  substances  by  increas- 
ing their  concentration.  The  application  of  heat  to 
the  electrolyte  is  permissible  when  no  undesirable 
secondary  reactions  and  no  appreciable  expense  are 
incurred.  As  a  rule,  the  liquors  are  sensibly  heated 
by  the  current  itself. 

In  experiments  with  organic  substances,  it  may  be 
added  that  they  must,  in  all  cases,  be  dissolved  before 
they  are  subjected  to  electrolysis.  When  they  are 
insoluble  in  water,  they  should  be  brought  into  a 
suitable  condition  for  the  action  of  the  current  by 
means  of  acids,  alkalies,  etc. 

An  advantage  in  many  electrolytic  processes  is 
found  in  maintaining  a  thorough  agitation  of  the  elec- 
trolyte; this  can  be  effected  by  circulation  or  by 
mechanical  means.  It  is  in  electrometallurgy  par- 
ticularly that  a  more  or  less  complete  circulation  of 
the  liquor  plays  an  important  role. 


THE   ARRANGEMENT   OF   EXPERIMENTS.  75 


THE  ARRANGEMENT   OF  EXPERIMENTS. 


FIG.  16. 


The  regulating  resistance  R  is  first  thrown  into  the 
circuit.  It  is  followed  by  A — an  apparatus  intended 
to  measure  the  current  strength.  This  is  either  a 
technical  amperemeter,  a  galvanometer,  or  the  shunt 
of  a  galvanometer.  If  the  experiment  is  to  be  car- 
ried out  quantitatively,  the  ampere-hour  meter  is  next 
inserted;  it  is  the  copper  voltameter  K.  The  current 
is  then  introduced  and  passes  through  the  experi- 
mental cell  V,  the  screw-clamp  U,  used  to  interrupt 
the  current,  and  then  back  to  the  battery.  To  ascer- 
tain the  pressure  in  the  experimental  cell  the  instru- 
ment S  is  attached  to  the  two  electrodes  by  means  of 
a  thin  copper  wire.  This  instrument  is  either  a  volt- 


76        ELECTROCHEMICAL  EXPERIMENTS. 


N 


FIG.  17. 


FIG.  18. 


PRESSURE   OF    DECOMPOSITION.  77 

meter  or  a  torsion  galvanometer,  provided  with  a  series 
resistance.  The  latter  instrument  can  be  used  both 
for  the  measurement  of  current  and  of  pressure,  by  at- 
taching its  conducting  wires  to  the  shunt-resistance 
N  or  to  the  series  resistance  W,  in  connection  with 
the  experimental  cell  (Figs.  17  and  18). 

All  such  aids  as  are  used  in  a  chemical  examina- 
tion of  the  precipitates,  solutions,  and  gases  arising 
in  an  electrolytic  experiment,  are  to  be  considered  as 
a  part  of  the  necessary  outfit. 


C.  PHENOMENA   ARISING   IN   ELECTRO- 
LYSIS. 

PRESSURE   OF  DECOMPOSITION.     POLARIZING 
CURRENTS. 

When  water  acidified  with  sulphuric  acid  is  elec- 
trolysed with  insoluble  anodes  and  a  varying  electro- 
motive force,  it  will  be  observed  that  its  decomposi- 
tion begins  when  the  pressure  =1.5  V.  Water 
can  not  be  resolved  into  its  components — hydrogen 
and  oxygen — with  a  lower  pressure.  The  higher 
this  factor  rises,  the  more  energetic  will  the  current 
be.  As  in  the  case  of  water,  so,  too,  in  every  other 
chemical  compound,  there  is  a  certain  minimum  pres- 
sure, below  which  decomposition  does  not  occur. 
When  hydrogen  and  oxygen  combine  to  yield  water, 


78         ELECTROCHEMICAL  EXPERIMENTS. 

heat  is  evolved,  energy  is  dissipated.  The  water 
molecule  has  less  energy  than  its  components  pos- 
sessed when  they  were  free  and  before  they  united. 
If,  then,  the  water  molecule  is  to  be  decomposed,  this 
will  only  be  possible  by  applying  an  amount  of  en- 
ergy equal  to  that  which  is  lost  in  its  formation. 
This  energy  is  supplied  by  the  electromotive  force  of 
the  current.  The  law  of  the  conservation  of  energy 
would  lead  to  the  hope  that  the  decomposition- 
pressure  might  be  calculated.  This  is,  indeed, 
possible. 

The  heat  modulus  (thermal  value)  is  the  number  of 
calories,  in  grams,  set  free  in  the  formation  of  a  mole- 
cule of  a  chemical  compound  referred  to  one  gram  of 
hydrogen  as  a  unit.  The  thermal  value  of  water  is, 
therefore,  -f-  68, 360  c. :  /.  e.,  in  the  union  of  2  grams  of 
hydrogen  with  16  grams  of  oxygen,  forming  18  grams 
of  water,  heat  is  evolved  sufficient  to  raise  68,360 
grams  of  water  i°  C.  This  is  expressed  symbolically 
by  placing  a  comma  between  the  symbols  of  the 
several  elements  and  enclosing  the  whole  in  brackets  : 

(H2,0)  =  +  68,360. 

Water  of  solution  is  represented  by  "  aq."  This  is 
always  very  large  in  proportion  to  the  dissolved  sub- 
stance. (KCl  aq)  represents  the  aqueous  solution 
of  a  molecule  of  potassium  chloride. 

Knowing  the  thermal  value  (heat  modulus)  attached 
to  an  electrolytic  process,  the  decomposition-pressure 


POLARIZING   CURRENTS.  79 

may  be  calculated  in  the  following  manner  :  *  Divide 
the  thermal  value  (heat  modulus)  W  by  the  product 
of  23,067  multiplied  by  the  number  of  valences  dis- 
solved by  the  current: 


Examples  : 

1.  In  the  case  of  (H2,O): 

W  =  68,360. 
n    =  2, 

68,360 

therefore  Z  =  -         r  =  1.48  V. 
2  .  23,067 

2.  In  the  decomposition    of  copper   sulphate  (Cu 
SO4,  aq),  with  insoluble  anodes,  into  copper,  oxygen, 
and  sulphuric  acid  (Cu,O,SO3  aq)  we  find 

W  =  55,960 
n    =  2, 

55,960 
consequently  Z  =  -          r  =  1.21  V. 

*  2  .  23,067 

3.  In  the  decomposition  of  (AgCl)  into  (Ag,Cl): 

W  =-.  29,380 
n  =  I, 

hence  Z  =  1.27  V. 

If  the  current  supply  in  an  electrolysis  be  inter- 
rupted and  a  galvanometer  be  connected  with  the 
electrodes,  it  will  be  observed,  from  the  deflection, 

*  The  deduction  of  this  formula  is  given  in  any  complete  volume  on 
Physics. 


8O  ELECTROCHEMICAL    EXPERIMENTS. 

that  a  current  proceeds  from  the  electrodes,  which, 
as  regards  direction,  is  the  opposite  of  that  which 
was  originally  sent  through  the  electrodes.  At  first 
it  will  be  quite  strong,  but  it  will  gradually  grow 
less.  This  phenomenon  has  long  been  known  under 
the  name  of  "current  polarization."  It  is  attribu- 
table or  due  to  the  ions,  separated  at  the  electrodes, 
seeking  to  re-unite  to  the  original  compound. 

The  current  produced  in  this  way  must  manifestly 
be  opposed  to  the  original  current.  If  the  products 
of  electrolysis  are  not  removed,  should  they  remain 
in  the  solid  or  liquid  condition  in  the  vicinity  of  the 
two  electrodes,  the  polarization  current  can,  depend- 
ing upon  its  quantity,  enrich  the  original  primary 
current  very  materially  (storage  cells).  If  gas  evo- 
lution has  occurred  at  one  or  both  of  the  electrodes, 
the  polarization  current  can  carry  only  a  fraction  of 
the  primary  current.  At  first,  so  long  as  the  elec- 
trodes are  covered  by  a  gaseous  layer,  this  current 
is  quite  powerful,  but  it  diminishes  rapidly.  The 
polarization  current  is  also  responsible  for  another 
phenomenon.  If  primary  batteries  are  used  in  carry- 
ing on  an  electrolytic  process, — a  process  well  adapted 
from  the  preceding  statements  for  the  production  of  a 
continuous  polarization  current,— it  may  occur  in  time 
that  the  electromotive  force  of  the  elements  will  be 
exhausted  and  become  equal  to  that  of  the  polariza- 
tion current  Then,  equilibrium  would  be  established 
between  the  battery  and  the  experimental  cell,  with 


POLARIZING   CURRENTS.  8 1 

the  consequent  cessation  of  current.  If,  for  any 
reasons,  the  pressure  of  the  battery  continues  to  fall 
even  lower,  the  direction  of  current  will  be  reversed ; 
the  experimental  cell  will  discharge  itself,  like  a  stor- 
age cell,  into  the  battery.  A  like  occurrence,  dis- 
charge of  the  experimental  cell,  with  annihilation  of 
all  that  has  already  been  accomplished,  will  be  ob- 
served, if,  when  a  dynamojs  employed  for  the  source 
of  current,  the  circuit  is  not  interrupted  at  the  close 
of  the  work.  The  armature  then  acts  like  a  short 
circuit.  The  polarization  current  can,  under  certain 
conditions,  become  so  powerful  that  it  will  be  strong 
enough  to  again  set  the  inactive  machine  into  motion. 

The  decomposition-pressure  previously  mentioned, 
is  also  the  maximum  electromotive  force  of  the  polari- 
zation current.  It  can  usually  be  determined  by  permit- 
ting the  electrolysis  to  proceed  for  some  time,  then  in- 
terrupting the  source  of  current  and  as  quickly  as 
possible  attaching  the  torsion  galvanometer.  In  this 
manner  it  may  also  be  demonstrated  that  the  electro- 
motive force  of  the  polarization  current  gradually 
grows  less. 

When  soluble  metallic  anodes  are  employed  in 
an  electrolysis,  the  conditions  become  complicated. 
The  anion — the  constituent  deposited  at  the  anode 
— is  not  liberated  as  such,  but  combines  in  statu  nas- 
cendi  with  the  substance  of  the  anode.  This  union 
develops  heat  which  is  favorable  to  the  process. 
Consequently,  the  current  is  not  called  on  to  furnish 


82         ELECTROCHEMICAL  EXPERIMENTS. 

all  the  energy  required :  it  is  only  necessary  that  this 
should  equal  the  difference  between  the  two  thermal 
values  (heat  moduli),  i.  e.,  the  heat  of  decompo- 
sition of  the  electrolyte  minus  the  heat  of  formation 
of  the  new  compound  produced  at  the  anode.  If 
the  anode  consists  of  the  same  metal  as  that  which 
is  to  be  deposited  from  a  solution,  e.  g.,  as  in  a 
copper  or  silver  voltameter,  then  the  pressure  of 
decomposition  becomes  zero,  for  the  energy  con- 
sumed at  the  kathode  in  the  decomposition  of  copper 
sulphate  into  Cu  -f-  SO4  is  just  as  great  as  that  pro- 
duced at  the  anode  by  the  union  of  SO4  with  Cu. 
When  the  pressure  of  decomposition  equals  zero, 
naturally  there  can  be  no  polarization  current;  hence 
it  is  said  of  such  combinations  that  "  the  polarization 
is  removed  by  the  choice  of  soluble  anodes."  In  prac- 
tice, the  decomposition-pressure  never  entirely  disap- 
pears, but  is  only  more  or  less  reduced,  hence  the 
polarization  current,  arising  from  the  reaction,  may 
be  constantly  detected. 

Attention  may  here  be  called  to  a  misunderstand- 
ing which  the  author  has  frequently  encountered. 
We  read,  and  sometimes  hear,  that  the  polarization 
may  be  removed  by  blowing  a  current  of  air  into  the 
experimental  cell.  If  a  gas  is  developed  in  a  pro- 
cess the  scheme  proposed  can  accomplish  merely  this, 
viz. — that  after  the  short  circuit  of  the  cell  only  a  very 
small  fraction  of  the  current  first  introduced  reap- 
pears as  polarizing  current.  This  is  because  the  one 


POLARIZING   CURRENTS.  83 

product  of  the  electrolysis — the  gas — is  removed, 
therefore  the  material  for  the  prolongation  of  the  po- 
larizing current  is  absent.  It  is  a  mistake,  however, 
to  suppose  that  by  the  mere  blowing-in  of  air  it  is 
possible  to  obtain  a  lower  decomposition-pressure. 
The  chemical  reaction  which  the  current  completes, 
is,  as  has  been  observed,  ample  for  this  purpose,  and 
this  is  in  no  sense  influenced  by  blowing  in  air. 

A  second  special  case  arises  in  the  use  of  soluble 
anodes,  consisting,  not  of  metal,  but  of  some  chem- 
ical compound.  As  a  typical  case,  the  process  of 
Marchese*  may  be  here  introduced.  In  it  plates 
of  copper,  serving  as  kathodes,  are  hung  in  a 
solution  of  blue  vitriol,  while  the  anodes  consist  of 
plates  made  from  copper-matte.  The  latter  is  mainly 
cuprous  sulphide  (Cu2S),  with  a  small  amount  of 
iron  sulphide  (FeS).  The  following  reactions  occur : 
Copper,  from  the  blue  vitriol  solution,  separates  at 
the  kathode.  It  has  the  thermal  value  (Cu,SO4  aq). 
Sulphur  appears  at  the  anode,  while  copper  dis- 
solves as  sulphate — CuSO4.  The  course  of  the 
reaction  may  be  imagined  to  progress  in  this  way : 
At  first  the  copper  sulphide  breaks  down  into 
sulphur  and  copper;  and  for  this  change  a  quantity 
of  energy  corresponding  to  the  thermal  value  (Cu2,S) 
will  be  requisite.  After  this  the  liberated  copper  will 
be  converted  into  copper  sulphate,  and  in  so  doing 

*D.  R-P.,  Nr.  22429. 


84         ELECTROCHEMICAL  EXPERIMENTS. 

produces  energy  corresponding  to  the  thermal  value 
(Cu,SO4  aq).  Since  the  thermal  value  of  the  electro- 
lysis is  equal  to  the  algebraic  sum  of  the  thermal 
values  developed  at  the  two  electrodes,  we  then  have, 
if  the  heat  consumption  be  represented  by  —  ,  and 
the  heat  production  by  +, 

KATHODE  ANODE 

—  (Cu,S04  aq)  —  (Cu,S)  +  (Cu,SO4  aq). 

The  first  and  third  members  fall  away,  so  that  in 
the  process  the  actual  energy-quantity  is  that  corre- 
sponding to  the  decomposition  of  the  copper  sul- 
phide. As  (Cu2,S)  equals  20,270  c,  the  decomposition- 
pressure  will  be 


.. 

2  .   23067 

This  diminution  in  the  pressure  of  decomposition 
only  occurs  when  the  material  to  be  dissolved  is  a 
conductor  of  electricity  and  serves,  at  the  same  time, 
as  anode,  so  that  the  solution  occurs  in  consequence 
of  the  direct  action  of  the  current.  For  example, 
this  end  would  not  be  attained  if  insoluble  anodes 
were  used,  and  the  ore  were  to  be  suspended  in  the 
bath  by  means  of  baskets.  A  solution  of  the  ore 
occurring  in  this  way,  if  indeed  it  did  happen,  would 
be  a  secondary  reaction,  which  would  have  absolutely 
nothing  to  do  with  the  electrolysis,  hence  would  not 
affect  the  required  pressure. 

Recently  several  suggestions  have  been  made  for 
obtaining  metals  electrolytically  which  seek  to  reduce 


POLARIZING   CURRENTS.  85 

the  pressure  of  decomposition  by  a  very  peculiar 
method.  Thus,  insoluble  anodes  have  been  used  and 
these  are  hung  in  the  solution  of  some  readily  oxi- 
dizable  substance.  The  electrodes  are,  therefore, 
separated  from  one  another  by  a  diaphragm.  Indeed, 
Siemens  (D.  R.-P.,  42243  and  48959)  electrolyzed  a 
mixed  solution  of  ferrous  sulphate  and  copper  sul- 
phate by  allowing  the  solution  first  to  pass  through 
the  kathode  chamber,  and  then  through  the  anode 
compartment.  In  this  way  copper  was  deposited  at 
the  kathode,  after  which  the  solution  of  ferrous  sul- 
phate passed  to  the  anode  where  it  took  up  the 
atomic  group  SO4 — the  anion  separated  from  the 
copper  sulphate — and  became  ferric  sulphate.  The 
latter  was  next  digested  with  finely  divided  chalco- 
pyrite  or  copper-matte.  Sulphur  separated,  the  ferric 
salt  was  reduced,  and  copper  sulphide  dissolved,  thus 
regenerating  the  original  liquid,  which  in  turn  was 
again  subjected  to  electrolysis. 

The  thermochemical  changes  were : 

ANODE  KATHODE 

+  (2FeS04aq,S04)  —  (Cu,SO4aq). 

The  thermal  values  needed  here  are  not  exactly 
known.  The  difference  in  question  can,  however,  be 
calculated  by  writing  the  reactions  differently : 

+  (2FeS04aq,0,S03aq)  -  (Cu,O,SO3aq). 

The  heat  of  oxidation  resulting  from  the  conver- 
sion of  ferrous  into  ferric  sulphate  may  be  ascertained 


86         ELECTROCHEMICAL  EXPERIMENTS. 

by  subtracting  the  heat  of  formation  of  two  molecules 
of  ferrous  sulphate  from  the  thermal  value  (heat 
modulus)  of  ferric  sulphate  : 

(Fe2,O3,3SO3aq)  =  224,880  c. 
—  2(Fe,O,SO3aq)  =  —  186,460  c. 


(2FeSO4aq,O,SO3aq)  =  38,4800. 

Hence  the  heat  consumption  at  the  kathode  (Cu,O,- 
SO3aq)  equals  55,960,  the  heat  produced  at  the  anode 
38480,  and  the  difference  would  be  17,480  c,  which 
would  correspond  to  a  pressure  of  0.38  V. 

In  fact,  the  author,  experimenting  on  a  small  scale 
and  observing  the  precautions  laid  down  on  p.  80, 
was  able  to  confirm  a  pressure  of  decomposition 
equal  to  0.38—0.41  V.  by  using  platinum  anodes,  and 
on  substituting  anodes  of  carbon  found  0.37-0.40  V. 
A  sheet  of  lead,  used  as  anode,  gave  an  evolution  of 
oxygen  and  a  decomposition  pressure  of  1.26  V. 
This  also  occurred  when  the  experiments  were  varied 
in  every  possible  way.  Therefore  Siemens'  process 
is  only  practicable  with  platinum  or  carbon  anodes, 
but  not  with  lead  anodes  !  There  is  no  explanation 
of  this  rather  singular  deportment. 

Hopfner  (D.  R-P.,  53782),  using  cuprous  chloride, 
and  Seegall  (D.  R-P.,  53196)  employing  ferrous  chlo- 
ride, pursued  an  idea  similar  to  that  of  Siemens.  The 
suggestion  that  oxidizable  salts  should  be  used  in 
electrolysis  is  deserving  of  great  consideration. 

As  observed  in  the  Siemens  process,  the  nature  of 


POLARIZING   CURRENTS.  8/ 

the  electrodes  also  exercises  an  influence  upon  the 
pressure.  An  example  will  show  this.  When  a 
copper  sulphate  solution  is  electrolyzed  with  plati- 
num anodes  the  pressure  of  decomposition  will  be 
observed  to  be  1.26  V.  On  substituting  lead  anodes 
these  will  gradually  become  coated  with  dioxide 
(PbO2)  and  on  making  a  test,  the  pressure  of  decom- 
position will  be  found  to  be  1.55-1.60  V.  This  is 
due  to  the  reactions  which  give  rise  to  the  polariza- 
tion current.  The  reverse  electrolysis  takes  place 
with  platinum  anodes,  while  with  lead  anodes  there 
comes  into  play  the .  formation  of  lead  sulphate 
(PbSO4)  from  the  oxide  (PbO)  and  the  sulphuric  acid 
(H2SO4).  The  dioxide  at  the  anode  first  loses  an 
atom  of  oxygen  and  passes  into  oxide  (PbO),  which 
unites  with  the  free  sulphuric  acid  to  form  lead  sul- 
phate and  water.  In  fact,  the  heat,  resulting  from  the 
formation  of  the  lead  dioxide  (PbO2)  from  lead  (Pb) 
and  oxygen  (O2)  should  tend  to  diminish  the  pressure 
of  the  primary  electrolysis;  yet  nothing  of  this  kind 
is  observed,  because  the  main  reaction  evolves  oxy- 
gen, and  but  little  of  it  is  consumed  in  the  formation 
of  PbO2.  With  the  polarizing  current,  however,  the 
gradually  accumulating  dioxide  immediately  comes 
into  evidence  with  its  entire  mass. 

The  magnitude  of  the  pressure  of  decomposition 
and  its  eventual  diminution  by  means  of  aid-reactions, 
are  of  the  greatest  importance  for  the  commercial 
success  of  a  process  or  method.  In  the  case  of  a 


88         ELECTROCHEMICAL  EXPERIMENTS. 

metallic  conductor  the  pressure  E,  which  is  necessary 
to  preserve  a  current  of  I  ampere  in  a  resistance  w, 

equals : 

E  =  I.  w. 

In  the  electrolysis  there  is  added  to  this  the  pres- 
sure of  decomposition  Z,  so  that  for  every  electro- 
lysis the  necessary  pressure  is 

E=:  I.   W   +    Z. 

In  this  expression  Z  is  a  constant,  determined  only 
by  the  nature  of  the  electrolytic  process  and  wholly 
independent  of  the  dimensions  of  the  bath.  The 
value  I.  w.,  however,  is  most  intimately  connected 
with  the  dimensions  of  the  bath  and  is  smaller,  the 
lower  the  resistance  w  of  the  bath.  Consequently  Z 
is  usually  greater  than  I.  w.  and  will,  in  large  meas- 
ure, determine  the  pressure  necessary  for  carrying  on 
the  work  of  the  bath.  The  higher  the  pressure,  how- 
ever, the  greater  will  the  energy  consumption  for  a 
definite  production  be.  Hence,  in  every  electrolytic 
process,  care  must  be  exercised  to  have  the  pressure  of 
decomposition  as  low  as  possible,  i.  e.,  use  soluble 
anodes  whenever  it  is  possible,  or  if  this  be  not  prac- 
ticable, endeavor  to  facilitate  the  electrolytic  process 
by  other  aid-reactions. 

FARADAY'S  LAW.    CURRENT  EFFICIENCY. 

When  a  current  is  passed  through  different  solu- 
lutions  in  series,  the  products  separated  in  equal  peri- 


FARADAY'S  LAW.     CURRENT  EFFICIENCY.         89 

ods  of  time   are    in   proportion   to   their  equivalent 
weights — (law  of  Faraday). 

For  example,  the  same  current  in  an  equal  period 
of  time,  decomposes 

I  molecule  of  HC1,  AgNO3,  KOH  ; 
y,         »       "  ZnCl2,  CuSO4,  H2O; 
yz         "       «  AuQ3,  NH3,  etc. 

As  a  current  of  one  ampere  precipitates  4.02  grams 
of  silver  per  hour,  it  is  easy  to  calculate  the  number 
of  grams  of  any  substance  which  will  be  decomposed 
or  deposited  in  a  definite  period  of  time. 

Example.  How  much  blue  vitriol  can  be  decom- 
posed daily, in  a  bath,  using  a  current  of  50  amperes? 
How  much  copper  will  be  deposited  ? 

In  accordance  with  the  law  just  stated  above,  for 
every  atom,  or  107.6  parts  by  weight  of  silver,  one- 
half  molecule  =  124.6  parts  of  CuSO4,5H2O  will 
be  decomposed,  and  consequently  j£  Cu  =  31.6 
parts  by  weight  of  copper  will  be  deposited.  One 
ampere-hour  corresponds  to  4.02  grams  of  silver, 

hence  4.02  .  — '—  =  4.65  grams  of  blue  vitriol  and 

4.02  — '—  =  1. 1 8  grams  of  copper;  so  that  in 

24  hours,  with  a  current  of  50  amperes  :  24  .  50  .  4.65 
—  55^°  grams  of  CuSO4,5  aq  will  be  decomposed 
and  there  will  separate  24  .  50  .  1.18  =  1416  grams  of 
copper. 

When  a  metal  enters  its  compounds  with  varying 


9O  ELECTROCHEMICAL    EXPERIMENTS. 

valence,  the  quantities  of  that  metal  deposited  by 
the  current  will  naturally  vary  according  to  the  va- 
lence. With  the  same  current,  Cu2Cl2  will  yield  twice 
as  much  copper  as  CuSO4.  In  most  instances  the 
reductions  first  proceed  to  the  lowest  state  of  oxida- 
tion before  the  metal  begins  to  separate. 

Very  few  electrolytic  processes  proceed  in  a  simple 
way.  In  most  cases  secondary  reactions  play  an 
important  role.  There  is  a  double  possibility  for  this  : 
unintentional  oxidations  can  occur  at  the  anode  or 
reductions  at  the  kathode.  In  such  instances  the 
law  of  Faraday  serves  for  the  sum  of  the  reactions 
occasioned  by  the  current.  In  the  decomposition  of 
copper  nitrate,  metallic  copper  appears  at  the  kathode, 
while  a  portion  of  the  nitric  acid  is  reduced  to  lower 
oxides  of  nitrogen,  or  even  to  ammonia.  Oxygen  is 
liberated  at  the  anode,  and  is  partially  consumed  in 
the  reoxidation  to  nitric  acid  of  the  nitrogen  oxides 
formed  at  the  kathode.  When  ferric  chloride  is 
electrolyzed,  ferrous  chloride  and  metallic  iron  occur 
at  the  kathode,  which  is,  in  turn,  converted  at  the 
anode  into  ferric  chloride.  A  similar  phenomenon 
explains  why  manganese  can  not  be  quantitatively 
precipitated  at  the  anode  from  a  liquid  containing 
iron,  because  the  salts,  in  a  lower  state  of  oxidation, 
which  have  been  simultaneously  formed,  favor  the 
re-solution  of  portions  of  the  manganese. 

It  is  of  the  utmost  importance  to  know  the  quan- 
titative course  of  an  electrolytic  process.  That  por- 


FARADAY'S  LAW.     CURRENT  EFFICIENCY.         91 

tion  of  the  total  current  sent  into  a  bath,  which  is 
active  in  the  direction  desired,  represents  the  current 
efficiency,  while  the  remainder,  consumed  in  undesired 
secondary  reactions,  is  the  current  loss.  In  every 
electrolytic  process  the  secondary  reactions  should 
be  determined  both  qualitatively  and  quantitatively, 
and  by  alteration  of  conditions  the  effort  should  be 
made  to  eliminate  these  disturbing  factors.  The 
character  of  the  side  reactions  may  be  discovered  by 
a  qualitative  examination  of  the  electrolyte  after  the 
completion  of  the  experiment,  and  the  amount  of 
loss  by  a  direct  determination  of  the  current  yield 
(efficiency). 

To  this  end  insert  an  ampere-hour  meter  (a  copper 
voltameter)  in  the  circuit  in  series  with  the  experi- 
mental cell  (see  p.  52).  Fill  a  large  beaker  or  glass 
jar  with  copper  sulphate  (p.  40),  observing-  the  ar- 
rangement of  apparatus  and  the  size  of  the  kathode, 
as  indicated  on  p.  53.  When  the  experiment  is 
finished,  determine  the  increase  in  weight  of  the 
kathode,  and  by  Faraday's  law  calculate  how  much 
of  the  desired  product  should  be  present  in  the  ex- 
perimental cell.  The  actual  amount  is  learned  by 
weighing,  or  by  titration,  and  its  percentage  may 
then  be  calculated. 

Example.  In  the  electrolysis  of  a  solution  of  salt, 
after  a  certain  time,  650  c.c.  of  liquor,  containing  4 
per  cent.  NaOH,  were  obtained.  During  this  same 
period  30.4  grams  of  copper  had  been  deposited  in 


92         ELECTROCHEMICAL  EXPERIMENTS. 

the   copper  voltameter.     Calculate  the   current  yield 
(efficiency). 

According  to  Faraday's  law,  2  molecules,  or  So 
parts  by  weight  of  caustic  soda,  result  for  each  atom, 
or  63.2  parts  by  weight  of  copper;  hence,  for  the 

30.4  grams   of  copper   obtained   in   the  experiment, 

38.5  grams  of  NaOH  would  be  produced  (63.2  :  80  = 
30.4 :  x  =  38.48).     The  actual  yield  was 

650  . =  26.0  grams. 

100 

Hence  the  current  efficiency  would  be 
26 

—  ==  67.5  per  cent. 
3°-5 

The  rest  of  the  current,  equaling  32.5  per  cent,  was 
therefore  expended  upon  other  reactions. 


ION   TRANSFERENCE. 

In  electrolysis  the  components  of  a  compound  are 
not  only  liberated  from  their  molecular  union,  but 
they'undergo  a  rearrangement.  If  the  movement  of 
the  liquid  particles  is  not  interfered  with,  this  phe- 
nomenon will  not  be  noticed,  but  if  it  is  restricted 
by  a  diaphragm,  alterations  in  concentration  will  be 
observed  at  the  electrodes. 

This  interesting  phenomenon,  which  can  not  be 
considered  theoretically  at  this  time,  may  be  best 
understood  by  an  example.  Decompose  dilute  sul- 


ION    TRANSFERENCE.  93 

phuric  acid  (56.7  grams  acid  to  the  liter)  between  two 
semicylindrical  sheets  of  lead,  by  a  current  of  one 
ampere.  Electrolytic  gas  will  be  set  free,  and  at  the 
expiration  of  the  experiment  the  acid  will  show  the 
same  concentration  which  it  first  had.  A  porous  cup 
is  next  introduced  between  the  electrodes.  In  three 
hours  the  acid  in  the  cup  will  contain  63.25  grams 
of  sulphuric  acid  per  liter,  while  the  external  liquid 
will  contain  55.47  grams  per  liter  of  the  same  acid. 
After  two  additional  hours  the  contents  will  be  found 
to  be  68.77  grams  and  52.86  grams  respectively. 
Hence  a  transference  has  occurred  on  the  part  of  the 
sulphuric  acid,  toward  the  anode ;  and  it  has  been 
withdrawn  from  the  vicinity  of  the  kathode.  By  re- 
versing the  experiment,  it  will  be  observed  that  this 
phenomenon  is  independent  of  the  varying  sizes  of 
the  electrode  surfaces.  On  mixing  the  two  portions 
of  acid  there  will  result  an  acid  containing  57.1  grams 
per  liter.  Its  slight  difference  from  the  original  content 
is  due  to  the  acid  of  unknown  concentration  absorbed 
by  the  porous  cup.  Next,  take  the  smaller  sheet  of 
lead,  which  has  been  hung  in  the  porous  cup,  and 
let  it  serve  as  kathode  while  the  larger  is  made  the 
anode.  In  three  hours  the  concentration  will  be  as 
follows: 

in  the  porous  cup  ( — )  50.65  grams  H2SO4  per  liter 
outside  the  porous  cup  (-(-)  60.04      "          "         "     " 

Again,  the  acid  in  the  anode  chamber  has  been  en- 
riched. Similar  phenomena  invariably  show  them- 


94         ELECTROCHEMICAL  EXPERIMENTS. 

selves  if  diaphragms  are  used  in  the  work.  They 
must  be  considered  as  a  part  of  the  investigation 
because  they  influence  the  concentration  of  the 
liquors  either  favorably  or  unfavorably. 


CURRENT  DENSITY. 

The  quantity  of  the  products  formed  in  any  elec- 
trolytic process  accords  with  the  law  of  Faraday  and 
the  current  strength;  the  quality,  however,  as  well 
as  the  nature  of  the  reactions,  depends  in  many  in- 
stances chiefly  upon  the  size  of  the  electrode  surfaces. 
Current  density  is  the  current  strength  for  a  unit  of 
surface  of  an  electrode.  In  technical  operations  the 
unit  of  surface  is  the  sq.  my  but  in  scientific  researches 
it  is  ordinarily  the  sq.  dm.  Whether  a  metal  is  de- 
posited electrolytically  in  a  spongy,  crystalline,  or 
amorphous  form  depends  very  much  upon  the  cur- 
rent density  prevailing  at  the  kathode.  The  smaller 
the  electrode  surface  the  more  energetically  must  the 
action  of  the  current  show  itself.  Thus,  Bunsen, 
by  using  exceedingly  high  current  densities,  actually 
succeeded  in  precipitating  metallic  chromium  from  an 
aqueous  solution. 

In  working  with  soluble  anodes,  the  influence  of 
current  density  will  also  find  expression.  When  the 
current  density  is  low,  the  individual  constituents  of 
the  anode  will  dissolve  one  after  the  other.  Metals 
in  solution  will  be  precipitated  one  after  the  other,  in 


CURRENT    DENSITY.  95 

accordance  with  the  law  that  the  metal  which  devel- 
ops the  greatest  energy  in  its  solution  will  first  dis- 
solve, and  that  one  which  for  its  deposition  requires 
the  consumption  of  the  least  quantity  of  energy  will 
be  first  precipitated.  As  the  density  increases,  these 
reactions  take  place  almost  simultaneously.  From  a 
neutral  solution  containjjig  copper  and  zinc,  a  current 
of  low  density  will  first  throw  out  the  copper,  while 
with  higher  density  an  alloy  of  copper  and  zinc,  i.  e., 
brass,  will  be  deposited. 

In  an  electrolytic  process  in  which  metals  are  not 
precipitated,  but  where  gases  or  substances  soluble 
in  water  are  separated,  it  might  be  assumed  that  the 
current  density  was  of  no  moment.  This,  however, 
is  not  the  case.  If  there  is  a  possibility  of  producing 
different  stages  in  the  oxidations  or  reductions  occur- 
ring at  the  electrodes,  as,  for  example,  is  often  the 
case  with  organic  bodies,  the  higher  density  will 
always  produce  a  more  far-reaching  oxidation  or  re- 
duction than  the  current  with  low  density. 

In  addition  to  the  physical  character  and  the  chem- 
ical nature  of  the  depositions  which  are  formed,  the 
current  density  further  exercises  an  influence  on  the 
pressure  of  the  bath.  If,  in  arranging  for  an  experi- 
ment, the  size  and  distance  of  the  electrodes  from 
one  another,  as  well  as  the  nature  of  the  electrolyte, 
remain  unchanged  and  the  current  density  alone  be 
varied,  it  will  be  observed  that  the  bath  pressure  will 
increase  simultaneously  with  the  density  of  the  cur- 


96         ELECTROCHEMICAL  EXPERIMENTS. 

rent.  This  phenomenon  is  not  singular  or  strange 
because,  inasmuch  as  the  resistance  of  the  bath  is 
not  altered,  a  higher  current-strength  can,  according 
to  Ohm's  law,  only  be  attained  by  a  higher  pressure. 


THE  WORKING  PRESSURE. 

Although  the  dependence  of  the  pressure  upon 
various  circumstances  has  been  touched  upon  at 
many  points  in  the  preceding  chapter,  these  relations 
must  again  be  considered  from  a  common  point  of 
view. 

The  working  pressure  depends  : 

(1)  Upon  the  decomposition-pressure  developed  by 
the  thermal  value  of  the  process  under  consideration. 

(2)  Upon   the   sum   of  all   the    resistances    of  the 
bath. 

(3)  Upon  the  current  density  which  must  be  main- 
tained. 

Economy  demands  that  the  working  pressure  shall 
be  low.  Let  us  see  how  this  can  be  accomplished. 

(a)  The  decomposition-pressure  of  a  process  is 
dependent  upon  the  chemical  changes  producing  it: 
hence  it  can  not  be  changed  at  will.  However,  as 
indicated  upon  p.  82,  it  is  possible  in  many  instances 
to  bring  in  a  reaction  which  will  aid  the  electrolysis 
— one  which  will  produce  heat — and  thereby  dimin- 
ish the  consumption  of  external  energy — in  the  form 
of  electricity.  The  use  of  soluble  anodes  and  the 


THE    WORKING    PRESSURE.  97 

continuous  addition  of  oxidizable  substances  to  in- 
soluble anodes  may  also  be  mentioned  in  this  connec- 
tion. 

(b)  The    resistance   of  the   bath    may  be   reduced 
by  greater  concentration  of  solution,  the  addition  of 
a  free  acid,  if  this  is  advisable  or  allowable,  by  heat- 
ing the  electrolyte,  and  by  bringing  the  electrodes 
nearer    together.     In   arranging  the  electrodes  care 
should  be  exercised  that  short-circuiting  between  ad- 
jacent electrodes  is  not  only  avoided  but  is  impossi- 
ble.    The  resistance  in  a  liquid  can  be  diminished 
not  only  by  bringing  the  electrodes  nearer  together, 
but  also  by  increasing  their  surface,  because  thereby 
the  liquid  layer  to  be  traversed  by  the  current  will 
acqVire   a    larger   cross-section.     This    is,   however, 
scarcely  allowable  because  the  size  of  the  electrode 
surfaces  has  already  been  fixed  by  the  current  density 
most  favorable  for  the  operation.     A  slight  advantage 
can  be  obtained  by  having  the  electrodes  of  unequal 
size.     The  occasion   will   be   rare  when   the  current 
density  at  both  electrodes  must  have  a  definite  value. 
This    is   necessary   usually  for   only  one   electrode, 
either  the   negative  or  positive,  depending  upon  the 
nature  of  the  process.     Therefore,  the  other  electrode 
may  be  made  larger,  and  in  this  way  the  resistance  in 
the  liquid  will  be  diminished.     It  is  only  possible  to 
make  an  essential  gain  in  this  manner  when  the  two 
electrodes  show  decided  differences  in  size. 

(c)  When  the  density  and   composition  of  the  so- 


98         ELECTROCHEMICAL  EXPERIMENTS. 

lution  in  an  electrolytic  process  are  given,  the  pres- 
sure value  is  at  once  fixed  and  only  the  minor  details 
can  now  be  considered,  such  as  may  be  produced  by  a 
greater  or  less  distance  between  the  electrodes  and  a 
higher  or  lower  bath  temperature.  With  such  provi- 
sions the  pressure  can  no  longer  be  varied  at  will  with- 
out changing  both  the  current  density  and  the  separa- 
tion of  the  electrodes.  With  unchanged  solution  and 
unchanged  distances  between  the  electrodes,  there  is  of 
necessity  a  lower  density  for  a  lower  pressure,  and 
vice  versa. 


D.    THE     PRELIMINARY     EXPERIMENTS 
NECESSARY    FOR  AN   ELECTRO- 
CHEMICAL PROCESS. 

If  electricity  is  to  be  used  in  carrying  out  a  process, 
answers  should  be  given  to  the  following  questions : 

1.  What  are  the  most  favorable  conditions  for  the 
experiment? 

2.  What  expense  will  attend  the  process  ? 

The  experiment  should  not  be  conducted  on  too 
contracted  a  scale.  The  selected  electrodes  should 
have  I oo  sq.  cm.  surface  per  side.  This  is  particularly 
desirable  where  metal  precipitations  are  concerned. 
The  metallic  deposits  on  the  edges  of  the  electrodes 
are  generally  different  from  those  appearing  on  the 
middle  of  its  surface,  so  that  a  false  judgment  might 


PRELIMINARY    EXPERIMENTS.  99 

easily  follow  if  the  electrode  had,  so  to  speak,  only 
an  edge  surface. 

As  electrolyte  choose  a  solution  apparently  suit- 
able for  the  purpose,  and  at  first  vary  the  current 
density.  Note  the  pressures  which  are  produced, 
and  observe  the  physical  and  chemical  nature  of  the 
deposits ;  examine  the  secondary  reactions  both 
qualitatively  and  quantitatively.  Next  alter  the  com- 
position of  the  solution — let  it  be  more  dilute,  more 
concentrated,  neutral,  acid,  alkaline.  Change  the  salts 
until  a  suitable  solution  and  proper  current  density 
have  been  found.  When  the  most  favorable  condi- 
tions have  been  discovered,  make  a  few  preliminary 
experiments  upon  the  current  efficiency,  and  when  it 
is  possible  let  this  be  done  with  extremes  in  tempera- 
ture (high  and  low)  which  are  likely  to  figure  in  the 
practical  application.  During  the  course  of  the  ex- 
periment it  will  no  doubt  become  patent  that  the 
construction  of  the  apparatus  must  be  altered.  In 
all  such  changes  consider  the  question :  Will  this 
arrangement  prove  satisfactory  if  the  dimensions 
are  those  required  when  working  on  a  large  scale? 
Technical  experiments  should  not  be  performed  with 
apparatus  whose  principles  can  not  be  applied  practi- 
cally. Results  obtained  with  such  apparatus  have 
little,  if  any,  technical  value,  simply  because  they  can 
not  be  directly  introduced  into  practice. 

In  the  whole  course  of  the  experiments  such  fre- 
quent opportunity  is  offered  to  exercise  the  ingenuity 


IOO       ELECTROCHEMICAL  EXPERIMENTS. 

as  chemist,  physicist,  and  engineer,  that  it  is  unneces- 
sary to  speak  of  all  the  possible  conditions. 

When  the  most  favorable  experimental  conditions 
have  been  found  and  suitable  apparatus  has  been 
constructed,  advance  is  made  to  the  next  stage  of  the 
experiment  by  having  a  small  dynamo  act  for  several 
weeks  or  months  on  a  number  of  baths  arranged  in 
series.  A  more  extended  view  of  the  process  is  thus 
gained,  and  the  deficiencies  in  construction  appear 
which  in  the  case  of  smaller  apparatus  would  be  over- 
looked. When  the  faults  have  been  largely  removed 
and  the  plant  works  well,  a  calculation  for  larger 
conditions  can  be  undertaken  without  fear  of  stumb- 
ling upon  great  deceptions  or  mistakes. 


E.   CALCULATION    OF   THE    NECESSARY 
POWER.     CHOICE  OF  DYNAMOS. 

To  calculate  the  power  required  for  an  electro- 
chemical process  it  is  necessary  to  know  the  pressure 
and  current  efficiency  for  each  bath. 

Suppose  that  the  bath  pressure  equals  s  volts,  and 
the  current  yield  <r  per  cent.,  then  the  work  of  a  horse- 
power would  be 

V  V  amperes  r  -' 

I   HP  = — — ;  with  a  pressure,  therefore,  of  s 

volts  there  would  consequently  result  amperes 


CALCULATION    OF    THE    NECESSARY    POWER.         IOI 

to  the  horse-power.  If  a  grams  of  some  definite  sub- 
stance is  produced  per  ampere-hour  (see  Table  i),  then 

for  each  horse-power  hour  there  would   result  -  -  . 

a  grams.  Since  the  current  efficiency  is  never 
quantitative,  but  only  equal  to  <r  per  cent.,  the  value 

above  must  be  further  multiplied  by  —  and  then  it 
becomes  —  Finally,  a  correction  must  be 

S  .    100 

added  for  the  efficiency  v  of  the  dynamo.  In  the 
case  of  small  dynamos  v  =  75  to  80  per  cent.,  whereas 
with  larger  machines  and  normal  loads  this  value 
rises  to  93  per  cent. 

Hence  the  actual  yield  for  each  horse-power  hour  is 

736  a  .  ff  .  v 

grams;  or  introducing  the  limiting  values 

of  v — 75  per  cent,  to  93  per  cent. — given  above,  the 
production  p  for  a  horse-power  hour  becomes,  depend- 
ing upon  the  size  of  the  machine  used — 

552  a  684  a 

p  =  —    —  .  a  .  -     -to—    —  .  a  .   -     —  . 

S  100  S  100 

Having  thus  deduced  the  production  for  a  horse- 
power hour,  the  daily  production,  having  a  definite 
power  at  command,  can  easily  be  ascertained,  or  the 
reverse,  viz.,  the  power  requisite  to  manufacture  daily 
ICO  kg.  of  any  product. 

In  projecting  a  plant,  the  result  above  indicated 
will  not  be  sufficient.  Regard  must  also  be  had 


IO2        ELECTROCHEMICAL  EXPERIMENTS. 

to  the  mixing  and  transportation  of  the  liquor, 
to  the  working  of  the  adjunct  machinery,  etc. 
If  a  steam  boiler  is  necessary,  attention  must  be  paid 
to  the  consumption  of  steam  in  heating  the  liquor, 
and  at  least  to  the  heating  of  the  working-rooms  by 
steam  during  the  winter. 

As  regards  the  choice  of  dynamos,  it  may  be  said 
that,  so  far  as  its  efficiency  goes,  it  is  immaterial 
whether  medium  pressure  and  powerful  current  or 
high  pressure  and  low  current  be  employed.  How- 
ever, the  following  must  be  observed  :  If  the  nature 
of  the  process  in  any  manner  permits,  i.  e.,  if  the  baths 
are  alike  and  regularly  served,  they  should  be  ar- 
ranged in  series.  The  number  thus  introduced  into 

o 

the  circuit,  and  the  pressure  that  the  machine  will 
consequently  have,  will  depend  upon  whether  fre- 
quent disturbances  occur,  due  to  cleansing  of  the 
baths  or  electrodes,  fresh  charges,  etc.  In  electro- 
lytic copper  refining,  40  to  50  baths  are  arranged  in 
series,  but  with  a  less  active  plant  15  to  20  constitute  a 
very  respectable  number.  If  each  bath  requires  a 
pressure  of  s  volts  and  if  ;/  baths  of  this  kind  are  to 
be  used  in  series,  a  pressure  of  V  =  n  .  s  volts  will  be 
required  of  the  dynamo.  As  a  rule,  the  pressure  of 
the  machine  should  be  higher  than  that  calculated, 
because  by  constant  work  it  falls  in  consequence 
of  the  heating  of  the  armature,  and  also  because  of 
imperfect  contacts  here  and  there.  The  latter  can 
scarcely  be  avoided,  so  that  an  increased  pressure 


CHOICE    OF    DYNAMOS.  1 03 

is  necessary  to  maintain  an  undiminished  current 
strength.  When  the  entire  electromotive  force  of 
the  dynamo  is  not  needed,  the  excess  is  cut  off  by 
being  sent  through  a  shunt  regulator. 

A  reserve  dynamo  should  always  be  on  hand,  so 
that  in  case  of  repairs  to  the  running  machine  the 
action  of  the  current  need  not  be  interrupted  for  days 
or  weeks.  It  is  recommended  to  use  the  one  ma- 
chine for  day  work,  and  the  other  for  work  at  night. 
In  this  way  both  machines  are  better  protected  than 
if  they  be  run  until  some  part  or  parts  are  completely 
used  up.  By  an  arrangement  such  as  the  preceding, 
the  periods  required  for  cleaning  can  also  be  minim- 
ized, and  the  person  in  charge,  finally,  can  have  a 
better  oversight  of  the  dynamos.  Each  workman 
can  then  supervise  his  own  machine,  and  can  not  con- 
cgal  any  mistakes  which  may  have  occurred  in  its 
operation.  Again,  it  will  be  easy  in  this  way  to 
arouse  the  sense  of  honor  of  each  attendant  so  that 
he  will  strive  to  keep  his  machine  in  the  best  con- 
dition. 

In  addition  to  the  expense  incurred  in  obtaining 
the  power  it  is  necessary,  when  calculating  the  cost  of 
the  manufacture,  to  include*  interest  on  the  purchase 
sums  for  the  entire  plant  (buildings,  machinery,  instru- 
ments, baths,  conductors),  the  cost  of  chemicals  con- 

*  See  also  Berg-  u.  Hiittenmann.  Zeitung,  1893,  52- 


IO4        ELECTROCHEMICAL  EXPERIMENTS. 

sumed  in  the  process,  and  wages,  as  well  as  loss  in 
interest  due  to  the  storage  of  the  necessary  supplies 
of  crude  material. 


F.  PRACTICAL  PART. 

i.  CONSTRUCTION    AND    CALIBRATION  OF  A  TAN- 
GENT GALVANOMETER. 

A  very  simple  contrivance  shown   in  the  accom- 
panying Fig.  19,  and  answering  every  purpose,  may 


FIG.  19. 

be  made  by  any  experimenter.  It  requires  no  special 
description.  Notches  are  made  in  the  little  table 
at  the  points  where  the  wire  passes  through  it.  It 
must,  however,  have  margin  enough  to  support  a 
glass  cover,  intended  to  protect  .the  needle  from  air- 
currents.  The  needle  is  a  short  magnetic  steel  rod 
(a  knitting  needle).  That  the  angle  of  deflection  on 


CALIBRATION    OF    GALVANOMETER.  IO$ 

the  largely  divided  circle  may  be  conveniently  read, 
attach  to  the  needle  by  means  of  shellac  or  wax  a  thin 
hollow  glass  thread  filled  with  ink.  A  fiber  of  the 
finest  sewing  silk  will  answer  for  the  suspension  fila- 
ment. This  also  is  placed  upon  the  sealing  wax,  and  by 
momentary  contact  with  a  hot  wire  is  attached  to  the 
needle.  A  drop  of  wax  or  paraffin  brought  upon 
the  glass  tube  will  serve  to  balance  the  needle.  This 
device,  it  is  true,  is  very  primitive,  but  it  will  suffice 
for  many  purposes,  especially  when  it  is  only  in- 
tended to  obtain  an  approximate  idea  of  the  current 
strength,  or  whether  it  changes  during  the  experiment. 
Tangent  galvanometers  are  calibrated  by  bringing 
them  into  circuit  with  a  voltameter  (arrangement  of 
experiment :  battery  —  galvanometer  —  voltameter — 
battery)  and  observing  the  deflections  a±  and  a.2,  with 
varying  current  strengths,  I  and  I2.  According  to 

I  =  c  .  tang,   a, 

consequently 

tang,  a 

The  values  for  c,  deduced  from  the  two  experi- 
ments, must  agree  fairly  well.  Determine  their  mean 
and  calculate  a  table  for  the  different  deflections. 

EXAMPLE.  The  length  of  the  needle  is  28  mm.,  and 
the  diameter  of  the  circle  is  167  mm. 

Experiment  i.  In  six  minutes  0.0508  gram  of  me- 
tallic copper  was  obtained  in  the  copper  voltameter; 

H 


IO6        ELECTROCHEMICAL  EXPERIMENTS. 

the  deflection  in  the  galvanometer  averaged  12°. 
The  precipitation  of  copper  per  hour  would,  there- 
fore, be  0.508  gram,  and  as  1.181  grams  copper,  per 
hour,  correspond  to  I  ampere,  the  calculated  cur- 

0.508 
rent  strength  would  be  — —  =  0.430  ampere.     The 

constants  of  the  galvanometer  may,  therefore,  be  cal- 
culated from  the  equation — 

0.430 

c—  -          -  =  2.023. 
tang.  12° 

Experiment  2.  In  six  minutes  0.0963  gram  of 
copper  was  precipitated.  The  average  deflection  was 
22°.  Therefore  the  current  strength  was 

0.963 

I  =  —    —  =  0.815  ampere, 
1.181 

hence 

0.815 

c  =  -          -  —  2.017. 
tang.  22° 

hence 

The  mean  of  the  two  experiments  is  c  =  2.02. 
This  is,  at  the  same  time,  the  current  strength  for  the 
deflection-angle  45°  (tang.  45°  =  i).  A  table  for  the 
galvanometer  can  now  be  constructed : 

1°  deflection  :    I  =  2.02  tang.  1°  =  0.035  Amp., 

2°          "  I  =  2.02  tang.  2°  =  0.070      " 

3°          "  I  =  2.02  tang.  3°  =  0.106      " 

etc. 


CALIBRATION    OF    GALVANOMETER.  IO/ 


2.  CALIBRATION  OF   A  GALVANOMETER  BY 
MEANS  OF  A  SHUNT. 

If  it  is  desired  to  enlarge  the  capacity  of  this  very 
sensitive  instrument,  recourse  may  be  had  to  a  shunt. 
This  must  be  considered  and  arranged  as  a  part  of 
the  main  circuit.  As  all  sensitive  galvanometers  have 
a  great  number  of  turns,  and  as  these  are  usually 
wound  flat,  the  constants  can  not  be  established  by 
one  or  two  experiments,  but  must  be  determined  em- 
pirically. This  is  done  by  making  a  number  of  read- 
ings with  different  current  strengths,  the  results  being 
then  transferred  to  millimeter  paper  on  which  the 
current  strengths  appear  as  the  abscissas  and  the  cor- 
responding deflections  as  the  ordinates.  The  points 
fixed  in  this  manner  are  then  united  into  a  continuous 
curve,  from  which,  by  inverting  the  order,  current 
strengths  corresponding  to  all  deflections  can  be  ob- 
tained. 

The  lower  the  resistance  of  the  shunt,  the  stronger 
may  the  currents  be  for  which  the  instrument  is  avail- 
able. It  should  be  remembered  that  in  consequence 
of  the  sensitiveness  of  the  apparatus  the  needle  may 
be  appreciably  influenced  by  the  main  current  if  the 
galvanometer  be  set  up  in  too  close  proximity  to  the 
latter.  Whenever  possible,  place  the  galvanometer 
from  ^  to  I  m.  from  the  main  current.  Should  the 
galvanometer  always  occupy  the  same  position,  the 
disturbance  may  be  disregarded,  because  it  has 


io8 


ELECTROCHEMICAL  EXPERIMENTS. 


been  fully  allowed  for  in  the  curve  previously  con- 
structed. If,  however,  the  galvanometer  and  its  shunt 
are  to  be  used  in  different  localities,  it  is  quite  neces- 
sary to  remove  them  from  the  main  current  to  the 
distance  mentioned. 

When  a  delicate  galvanometer  can  not  be  had,  a  very 
simple  form  can  be  constructed  after  the  style  of  the 
tangent  instrument  described  in  the  preceding  chap- 


FlG.    20. 

ter.  Instead  of  the  single  copper  ring,  use  a  coil 
of  covered  copper  wire  having  numerous  turns. 
The  needle  may  be  large  in  proportion  to  the  diam- 
eter of  the  ring,  as  no  use  will  be  made  of  the  tangent 
law. 

Example.  A  delicate  instrument  provided  with  a 
bell  magnet  and  copper  damping  was  used  as  the  gal- 
vanometer (Hartmann  and  Braun,  Frankfort-on-the- 


CALIBRATION    OF    GALVANOMETER. 


ICQ 


Main)  ;  its  resistance  was  3.35  Q.  The  shunt  con- 
sisted of  I  m.  of  copper  wire  2  mm.  in  diameter,  at 
each  end  of  which  a  plate  of  metal  with  two  clamps 
was  attached  by  solder.  This  shunt  N,  a  copper 
voltameter  K,  and  a  resistance  W,  serving  to  vary  the 
current  strength,  were  arranged  in  series  in  the  same 
circuit.  The  galvanometer  G,  was  connected  with  the 
shunt  N  by  means  of  two  copper  wires,  I  m.  in  length 
(Fig.  20).  (These  two  connecting  wires  form  a 
part  of  the  branch  resistance,  hence,  in  subsequent 
measurements,  can  not  be  exchanged  for  any  other 
two  wires.) 

Produce  with  W  currents  of  different  strengths ; 
note  the  deflection,  and  weigh  the  copper  deposited  in 
a  unit  of  time  (6  minutes,  preferably,  as  they  equal 
^  of  an  hour). 


DURATION  OF 
EXPERIMENT. 

DEFLECTION. 

PRECIPITATED 
COPPER 

CURRENT 
STRENGTH. 

MINUTES. 

DEC. 

IN  GRAMS. 

AMPERES. 

6       .    . 

.      5-6.   .. 

.    .  0.0114  .    . 

.    0.096 

6  ... 

.    .      7-5  .-. 

.    .  0.0154  .    . 

.    0.130 

6  ... 

.    .    10.8  . 

.     .  0.0222   .     . 

.    0.187 

6.    .    . 

.   .    14.3  .    .    . 

.    .  0.0307  .    . 

-    0.259 

8  ... 

.     .      20.0  .     .     . 

.    .  0.0599  •    • 

•    °-379 

6  ... 

.     .      22.1   .     .     . 

.    .  0.0502  .    . 

.   0.423 

7  •    •    . 

.     .      24.5   .     .     . 

.    .  0.0661  .    . 

.   0.478 

6 

28  o 

.  o  0668 

o.  ^6^ 

6  .    .    . 

.     .      32.0  .     .     . 

.    .  0.0802  .    . 

.   0.676 

6  ... 

37.O  . 

.    .  0.0961  .    . 

.   0.810 

6  .    .    . 

.     .      43.0  .     .     . 

.     .  O.I2OO  .     . 

.     1.  012 

6  .    .    . 

.    .    49-0  .    .    • 

.    .  o.  1488  .    . 

•     1-255 

110 


ELECTROCHEMICAL  EXPERIMENTS. 


In  Fig.  21  the  preceding  current  strengths  appear 
as  abscissas,  the  corresponding  deflections  as  ordi- 
nates,  and  the  resulting  points  of  intersection  indicated 
by  crosses  form  the  connecting  curve  : 


50r 


Os 


FIG.  21. 


The  following  table  can  be  read  from  this  curve  as 
a  result  of  the  calibration  : 


O.I 

o.  2 

0-3 
0.4 

0.5 
0.6 
0.7 
0.8 
0.9 

I.O 

I.I 

1.2 
1-25 


DEFLECTIONS. 


16.3 
21.  0 

25- 
29. 

33- 

36.5 

39-5 

43-o 

46. 

48. 

49- 


INSTRUMENT    TO    MEASURE    PRESSURE.  I  I  I 


3.  CONSTRUCTION  OF  AN  INSTRUMENT  TO  MEAS- 
URE   PRESSURE. 

If  several  storage  cells  are  at  the  disposal  of  the 
operator,  the  galvanometer  used  in  the  preceding 
experiment  may  be  calibrated  so  as  to  measure  pres- 
sure. When  a  single  accumulator  has  lost  a  small 
part  of  its  charge,  its  pressure  may  be  accepted  as 
fairly  accurate  if  taken  as  2.0  V.  If  the  cell  be 
discharged  by  a  German  silver  wire,  the  resistance  of 
which  has  been  so  chosen  that  the  wire  never  becomes 
too  hot  and  the  current  allowed  for  the  cell  is  never 
exceeded,  there  will  exist  between  the  terminals 
of  the  German  silver  wire  a  pressure  of  two  volts ;  at 
the  middle  it  will  have  one  volt,  and  between  the  one 
terminal  and  the  first  quarter  0.5  volt,  etc.  If  a  wire 
I  m.  in  length  has  been  selected,  each  cm.  of  it  will 
correspond  to  Ti7  of  the  pressure  prevailing  at  the 
terminals.  Hence  it  will  only  be  necessary  to  suc- 
cessively connect  the  galvanometer  by  its  wires  to  the 
points  of  the  German  silver  wire  representing  o.i,  0.2, 
0.3,  etc.,V.  pressure,  and  note  the  deflections  in  order  to 
use  the  instrument  as  a  voltmeter  graduated  in  tenths. 
In  order  to  obtain  suitable  deflections  of  the  galvan- 
ometer it  must  be  provided  with  a  series  resistance 
the  size  of  which  will  depend  upon  the  pressure  and 
the  delicacy  of  the  instrument.  As  mentioned  on  p. 
54,  the  use  of  a  high  series  resistance  increases  or  en- 


112  ELECTROCHEMICAL    EXPERIMENTS. 

larges  the  applicability  of  the  instrument.  Hence, 
having  the  choice  between  a  less  delicate  galvanom- 
eter with  lower  series  resistance,  and  one  that  is  very 
delicate,  and  for  the  introduction  of  which  a  high 
resistance  will  be  necessary,  it  will  be  wise  to  choose 
the  latter. 

Example.  A  piece  of  fine  German  silver  wire,  2  m. 
in  length,  is  stretched  over  a  narrow  board,  which  at  its 
terminals  is  soldered  to  binding  screws,  connected  by 
stout  copper  wires  to  two  accumulators,  in  series,  hav- 
ing a  maximum  discharge  capacity  of  ten  amperes. 
As  the  resistance  of  the  wire  equals  6.67  Q,  there 
exists  in  it,  during  the'experiment,  a  current  strength  of 

-  =  0.6  ampere.  To  obtain  a  proper  angle  of  de- 
6.67 

flection  for  the  pressure  of  4.0  V.  prevailing  at  the  ter- 
minals use  a  series  resistance  of  about  550  Q.  Under 
the  stretched  wire  is  placed  a  scale  2  meters  in  length 
and  divided  into  40  equal  parts.  The  division  lines  are 
five  cm.  apart  and  the  pressures  are  written  at  these 
points.  The  calibration  from  Tx¥  to  -fa  V.  is  made  in 
such  a  manner  that  the  one  binding  screw  of  the  gal- 
vanometer is  connected  by  a  fine  wire  to  the  first 
binding  screw  of  the  German  silver  wire,  while  the 
other  binding  screw  with  the  inserted  resistance  W  is 
connected  with  a  long,  flexible  wire.  This  is  carried 
along  the  German  silver  wire  and  at  each  division  the 
corresponding  deflection  of  the  galvanometer  noted. 
Fig.  22  represents  the  arrangement  as  projected. 


INSTRUMENT   TO    MEASURE   PRESSURE. 


+ 

-\ 

,    1  .  ,  ,  1  ,  ,  ,  ,  1  1  , 

<     1    1    1    1    |    1    1    1    1    1   4-4- 
1.0 

<    *    1    '    »   1    »    |    1         1    1    |    1    1    1    1   1    1    1    1    1    1    i    1    1    1  * 

2.0    V-,                3'°                      4»o 

FIG.  22. 


The  values    obtained   when   W      =   about    550    U 


are: 


PRESSURES. 
o.i  V. 

0.2 

o-3 
0.4 

0-5 
0.6 
0.7 


DEFLECTIONS. 
2.1° 

4-5 
6.6 

8-7 
10.8 
12.8 
14.6 


3-6 
3-7 
3-8 
3-9 
4.0 


51.6 
52.3 
53-o 
53-7 
54-4 


114        ELECTROCHEMICAL  EXPERIMENTS. 

When  a  second  series  resistance  of  only  100  Q  was 
used  to  obtain  a  greater  accuracy  for  a  lower  ca- 
pacity, the  following  deflections  were  observed  : 

PRESSURES.  DEFLECTIONS. 


o.io  V.  12° 

0.15  17.2 

O.2O  22.5 

0.25  27.4 

0.30  31.7 


0.80  56.5 

0.*5  58.1 

O.QO  59-4 

0.95  60.7 

1. 00  61.8 

The  pressure  corresponding  to  each  degree  of  the 
galvanometer  may  be  ascertained  by  carefully  con- 
structing a  curve,  as  shown  on  p.  no,  from  the 
observations  made  and  then  taking  from  it  the  num- 
bers not  directly  observed. 

Series  resistances  may  be  constructed  by  winding 
covered  nickelin  or  rheotan  wires,*  of  0.2  mm. 
diameter,  upon  wooden  spools. 

Each  wire  has  an  approximate  resistance  of  15  Q 
per  meter.  Stout  copper  wires  should  be  soldered  to 
the  terminals  and  the  whole  apparatus  be  then  placed 

*  May  be  obtained  from  F.  A.  Lange,  Berlin  C.,  Seydelstrasse. 


, 

OF  THE 

UNIVERSITY 

REGULATING    RESISTANCE. 


in  a  box  or  under  a  glass  cover,  the  terminals  of  the 
copper  wire  alone  projecting  beyond  the  latter. 


4.  THE    CALCULATION    FOR    AND    CONSTRUCTION 
OF   A  REGULATING  RESISTANCE. 

A  battery  of  storage  cells  is  at  the  disposal  of  the 
chemist.  Each  cell  is  allowed  a  maximum  discharge 
rate  of  ten  amperes.  The  resistance  in  circuit  may  be 
so  regulated  that  only  a  portion  of  the  four  cells  need 
be  connected  for  pressure.  This,  however,  would  be 
a  crude  arrangement,  and,  furthermore,  the  cells  would 
be  discharged  unequally,  so  that  after  the  lapse  of  a 
certain  period  some  of  the  cells  would  be  exhausted, 
while  others  would  still  be  in  a  workable  condition. 
It  would  be  best,  if  possible,  to  utilize  all  the  cells  in 
every  experiment.  They  would  then  sustain  equal 
discharge,  and  could  subsequently  be  equally  charged. 
A  wire  is  therefore  run  from  the  battery  to  the  experi- 
ment table.  Somewhere  in  the  line  introduce  a  reeu- 

o 

lator,  which  will  enable  the  operator  to  reduce  the 
current  in  every  possible  way  and  without  making  any 
great  jumps.  When  several  experiments  are  to  be 
conducted  simultaneously  by  means  of  the  same 
battery,  throw  in  such  a  resistance  between  each 
individual  experiment  and  the  main  circuit.  The 
resistance  wires  are  of  such  a  size  that  the  passing 
currents  are  not  sufficient  to  ignite  them.  The  wires 
should  not  be  too  long  and  not  too  different  in  kind. 


Il6       ELECTROCHEMICAL  EXPERIMENTS. 

The  ordinary  arrangement  of  the  battery  is  such 
that  the  four  cells  are  placed  in  series.  Their  yield 
would  then  be  8  V.  and  10  amperes.  The  group 
arrangement  of  2  X  2  cells  with  a  yield  of  20 
amperes  and  4  V.  rarely  occurs.  The  maximum 
current  strength,  due  to  the  resistance  in  question 
is,  therefore,  20  amperes.  Disregarding  altogether 
the  resistance  in  the  experiment  under  consideration, 

the  20  amperes  appear  with  a  resistance  of  -  -  =   0.2 

Q  and   10  amperes   flow  in   a  circuit  where  the  re- 

g 

sistance  is  —  =  0.8  &.     The  wire  for  the  resistance 

10 

subdivisions  under  I  Q  should  be  so  selected  that 
it  may  safely  carry  20  amperes.  From  the  table 
on  p.  63  nickelin  wire  1.5  mm.  in  diameter  would 
answer.  If  it  is  desired  to  regulate  down  to  0.2 

o 

ampere,  a  total   resistance  of  —  =40  Q  would  be 

requisite.  Indeed,  with  40  @  it  is  possible  to 
get  even  below  this  point,  because  the  resistance  of 
the  conducting  wires  in  the  particular  experiment 
must  always  be  added,  and  its  decomposition-pres- 
sure must  be  deducted  from  the  8  or  4  V.  of  the 
battery. 

Selecting  the  subdivisions  0.25,  0.25,0.5,  i,  2,  2,  5, 
10,  20  Q,  the  corresponding  current  strengths  in  the 
separate  subdivisions,  and  the  diameter  of  the  nickel- 
in  wire,  suitable  for  the  same,  may  be  found  as 
follows : 


REGULATING    RESISTANCE.  I  \J 

SUBDIVISION.  CURRENT  STRENGTH.  DIAMETER  OF  WIRE. 

Up  to  I  G  20 — 10  amperes                     1.5  mm. 

2—  5   "  4—1.6      "                           0.7     " 

10 — 40  "  0.8 — 0.2      "                           0.3    " 

The  entire  regulating  resistance  then  arranges  it- 
self as  follows  : 

SUBDIVISION.  LENGTH  AND  SIZE  OF  NICKELIN  WIRE. 

0.25  Q  1.09  m. 

0.25  "  1.09  "      V       1.5  mm. 

0.5     " 

1.  " 

2.  "  1.92    "        }-        0.7 
2. 

5-          " 

10.       "  1.79  "     f      0.3 

20.       " 

The  first  of  these  wires  is  arranged  in  a  long  loop, 
provided  with  a  sliding  contact,  so  that  its  resis- 
tance can  be  changed  from  o.  to  0.25  Q  by  mere  altera- 
tion in  position.  Two  thin  copper  plates,  bound 
together,  answer  for  the  sliding  contact.  They  are 
pushed  over  the  wire  loop  as  indicated  in  Fig.  23,  a. 
The  remaining  wires  are  wound  in  spirals  and  at- 
tached to  a  frame,  each  of  their  ends  being  soldered  * 

*  Every  person  engaging  in  electrolytic  experiments  should  possess 
some  skill  in  soldering.  Contacts  which  are  to  act  well  for  long 
periods  are  best  prepared  by  soldering.  The  bright,  clean  terminals  of 
the  two  wires  are  wrapped  about  each  other,  moistened  with  a  solution 
of  zinc  chloride,  heated  in  the  flame  of  a  Bunsen  or  alcohol  lamp,  and  the 
heated  point  touched  with  a  thin  hammered  piece  of  tin,  until  the 
latter  melts  and  flows  smoothly  over  the  soldered  spot.  If  the  solder 
does  not  spread,  the  wires  are  again  moistened  with  "  solder-water," 


115        ELECTROCHEMICAL  EXPERIMENTS. 

to  a  piece  of  stout  copper  wire  which  dips  into  a  cup 
of  mercury. 

To  throw  out  a  resistance  subdivision,  unite  the 
corresponding  mercury  cups  with  a  copper  wire  hav- 
ing this  form 


FIG.  23. 


5.    WORKING    A    COPPER     LIQUOR     CONTAINING 
ARSENIC. 

Problem.     A    factory   produces    daily    5    cb.  m.  of 
liquor,  which  contains  40  grams  of  copper  as  sulphate 

and  the  manipulation  repeated.  With  practice  the  end  may  soon  be 
attained  with  ease.  Each  solder-point  must  be  washed  to  remove  any 
adherent  acid,  which  would  gradually  destroy  the  metal. 


COPPER    LIQUOR.  I  IQ 

and  10  grams  of  arsenic  as  arsenic  acid,  per  liter. 
Other  heavy  metals  are  present  only  in  small  quan- 
tity, while  chlorine  and  nitric  acid  are  absent.  The 
attempt  is  to  be  made  to  purify  the  copper  as  far  as 
possible  in  the  electrolytic  way,  and  to  separate  it  in  a 
marketable  form.  Make  an  estimate  of  the  probable 
cost  of  the  plant  with  the  necessary  power,  keeping  in 
view  its  future  expansion. 

It  is  a  fact  well  known  to  every  chemist  that  copper 
can  not  be  separated  quantitatively,  as  metal,  in  the 
electrolytic  way  from  solutions  containing  arsenic 
without  some  of  the  latter  being  carried  down  with  it. 
At  the  beginning  of  the  electrolysis  pure  copper, 
bright  red  in  color,  separates ;  but  it  gradually 
grows  paler,  and  in  a  comparatively  short  time  it  is 
steel  gray,  or  even  black,  from  the  co-precipitated 
arsenic.  The  bright  blue  color  of  the  solution,  at 
this  point,  is  evidence  that  the  liquid  is  not  yet  free 
from  copper. 

In  the  electrolytic  separation  of  two  bodies,  such  as 
are  presented  in  this  case,  two  considerations  must  be 
observed  :  the  proper  current  density  nx\A  the  agitation 
of  the  liquor.  When  the  current  density  is  high,  the 
velocity  with  which  chemical  reactions  must  occur  at 
every  point  of  the  kathode  will  leave  very  little  time 
for  the  current  to  select  from  among  the  separate 
bodies  in  the  solution,  so  that  two  or  even  more  of 
them  will  be  simultaneously  precipitated.  With  low 
current  density,  however,  the  possibility  of  such  a 


I2O        ELECTROCHEMICAL  EXPERIMENTS. 

selection  will  occur.  Hence  a  very  definite  relation 
between  the  copper  and  arsenic  content  of  a  solution 
exists  for  every  current  density  beyond  which  point 
the  copper  precipitated  will  contain  arsenic.  These 
limits  should  be  determined. 

The  liquid  at  the  kathode,  from  which  copper  has 
been  deposited,  becomes  specifically  lighter  and  rises 
to  the  surface.  This  upward  movement  of  liquid  at  the 
kathode  can  be  readily  shown  by  carrying  out  the  ex- 
periment in  a  glass  vessel  and  examining  the  solution 
by  transmitted  light.  The  liquid  rising  in  the  vicinity 
of  the  kathode  becomes,  in  consequence  of  the  dimin- 
ished copper  content,  relatively  richer  in  arsenic,  as 
compared  with  the  major  portion  of  the  solution. 
Unfavorable  mixture  conditions,  therefore,  exist  about 
the  kathode  unless  the  difference  is  destroyed  by 
constant  stirring.  Thorough  agitation  of  the  liquor 
becomes  necessary  in  proportion  as  the  kathode 
plates  are  longer. 

With  the  preceding  observations  before  us,  the 
course  of  the  investigation  is  clearly  indicated.  To 
obtain  as  much  pure  copper  as  possible,  the  liquor  must 
be  constantly  agitated.  It  must  also  be  determined, 
with  varying  current  densities,  at  what  copper-content 
of  the  liquor  the  experiment  should  be  interrupted  to 
prevent  the  deposition  of  arsenic.  The  current  effi- 
ciency and  the  bath  pressure  should  be  ascertained  by 
an  inserted  voltameter.  The  last  two  factors  form  a 
basis  for  the  calculation  of  the  requisite  power,  while 


COPPER    LIQUOR.  121 

the  most  favorable  current  density  determines  the 
necessary  size  of  plant. 

Execution  of  Experiments.  Two  paraffined  cigar 
boxes  will  answer  for  containing  vessels.  They  should 
be  1 1  cm.  in  depth  and  in  length  and  be  6  cm.  wide. 
The  one  contains  the  solution  to  experiment  upon, 
while  the  other  does  service  as  a  copper  voltameter 
(see  p.  40).  The  electrodes  of  the  experimental  cell 
consist  of  two  plates  of  lead  as  anodes,  between  which 
is  suspended  a  thin  plate  of  copper,  serving  as  kathode. 
Each  metal  plate  is  10  X  10  cm.  The  electrodes  of 
the  voltameter  should  be  a  thick  and  a  thin  plate  of 
copper  of  equal  size.  Agitate  the  solution  by  a  stirrer 
which  constantly  moves  two  glass  rods  back  and  forth 
between  the  electrodes  (30  revolutions  per  minute). 
A  horizontal,  not  vertical,  movement  is  preferable, 
because  in  actual  practice  the  former  is  cheaper.  With 
an  up-and-down  movement  the  weight  of  the  agitator 
must  be  raised  with  each  lifting ;  this  means  a  con- 
siderable consumption  of  energy.  With  a  to-and-fro 
movement  of  the  agitator,  however,  a  rake  can  be 
attached  to  a  bar  passing  over  firmly  attached  carry- 
ing-rolls. This  does  away  with  the  weight  of  the 
agitator,  and  the  work  required  for  stirring,  occa- 
sioned by  the  resistance  of  the  liquid  and  the  friction 
of  the  carrying  rolls,  is  reduced  to  a  minimum. 

The  current  from  two  storage  cells  is  made  to  flow 
through  a  regulating  resistance,  and  afterward  passes, 
in  regular  succession,  the  shunt  of  a  galvanometer, 
i 


122        ELECTROCHEMICAL  EXPERIMENTS. 

the  experimental  cell,  the  copper  voltameter,  and  then 
back  to  the  battery.  The  liquor  contained  39.29 
grams  of  copper  and  9.9849  grams  of  arsenic  per  liter. 
The  experimental  cell  contained  530  c.cm.,  corre- 
sponding to  20.822  grams  of  copper  and  5.291  grams 
of  arsenic. 

The  first  experiment  was  conducted  with  a  current 
I  =  1.257  amperes  (calculated  from  the  voltameter), 
the  kathode  surface  immersed  in  the  liquor  was  10  X 
8.4  cm.,  hence  the  surface  used  was  168  sq.  cm.,  and 

the  current  density  was  D  =  — ~—  =  o .748    ampere 

for  each  sq.  dm.  In  trade,  whole  numbers  are  pre- 
ferred ;  hence,  in  calculating  the  current  density,  the 
square  meter,  one  hundred  times  as  great,  is  taken 
as  the  unit.  The  current  density  in  the  preceding  case 
would  then  be  "  75  amperes  per  sq.  m."  The  pres- 
sure remained  nearly  constant  at  1.92  V. 

The  experiment  was  interrupted  at  the  close  of 
eight  and  one-half  hours.  The  precipitated  copper 
had  a  perfectly  bright  color,  and  weighed  12.109 
grams.  In  the  voltameter  12.613  grams  of  copper 
were  deposited,  consequently  the  current  efficiency 

was 

12.109 


12.613 


=  96  per  cent. 


After  two  additional  hours  the  plate  of  metal  in 
the  voltameter  had  increased  2.8925  grams  in  weight, 
while  the  sheet  in  the  experimental  bath  had  in- 


COPPER    LIQUOR.  123 

creased  2.820  grams.  Hence  the  carrent  efficiency 
was  97.49  per  cent.,  the  mean  current  strength  1.224 
amperes,  and  the  current  density  73  amperes  per  sq. 
m.  The  precipitate  was  perfectly  satisfactory  in 
every  respect. 

The  following  morning  the  experiment  was  con- 
tinued for  two  hours  more.  The  pressure  equaled 
1.95  V.  In  the  voltameter  2.8195  grams  of  copper 
had  been  deposited,  and  in  the  experimental  bath  the 
quantity  of  copper  equaled  2.7828  grams.  The  cur- 
rent strength,  therefore,  was  1.194  amperes,  the  cur- 
rent density  71  amperes,  and  the  current  efficiency 
98.7  per  cent. 

As  the  copper  now  began  to  take  on  an  earthy 
color,  it  was  assumed  that  the  limit  for  the  current 
density,  so  frequently  mentioned,  had  been  attained. 
The  copper  precipitated  in  all  was 

12.109  grams. 
2.820      " 
2.783      •« 


Total,    17.712      " 

hence,  there  still  remained  in  solution  20.822  — 
17.712  =  3.11  grams  of  copper  and  5.29  grams  of 
arsenic,  or,  as  these  quantities  were  dissolved  in  530 
c.c.  of  liquor,  the  latter  contained,  per  liter,  5.87 
grams  of  copper  plus  9.98  grams  of  arsenic — i.  e., 
in  round  numbers,  6  grams  of  copper  for  10  grams  of 
arsenic. 


124        ELECTROCHEMICAL  EXPERIMENTS. 

Again,  a  new  plate  of  copper  was  suspended  in  the 
solution.  The  experiment  was  then  continued  with 
a  more  feeble  current,  consequently  with  a  lower  cur- 
rent density.  At  the  expiration  of  two  hours  there 
was  an  evident  gray-colored  deposit  of  arsenic.  The 
observations  were  as  follows  : 

The  copper  deposited  in  the  voltameter  was  1.4485 
grams. 

The  copper  deposited  in  the  experimental  cell  was 
I-3974  grams. 

Pressure  equaled  1.85  V. 

The  current  strength,  deduced  from  these  data, 
equaled  0.613  ampere;  the  current  density  equaled 
36.5  amperes  per  sq.  m.,  and  the  current  efficiency  was 
96.47  per  cent.  The  liquor  still  contained  3.11  —  I  40 
—  1.71  grams  of  copper,  together  with  5.29  grams  of 
arsenic,  or  in  every  liter  there  remained  3.23  grams 
of  copper  plus  10  grams  of  arsenic. 

The  gray-coated  metallic  plate  was  now  replaced  by 
another,  and  the  current  was  still  further  diminished 
to  0.25  ampere  for  a  kathode  surface  of  144  sq.  cm. 
The  current  density,  calculated  from  this,  equaled  18 
amperes  per  sq.  m.  The  pressure  was  1.69  V.  After 
three  hours  the  copper  precipitate  was  still  a  beau- 
tiful red  in  color.  The  deposited  quantities  were 
0.9089  gram  and  0.8354  gram,  from  which  the  cur- 
rent efficiency  of  92  per  cent,  was  calculated. 

At  the  end  of  another  hour  and  seven  minutes  the 
copper  became  coated  with  a  gray  deposit  of  arsenic, 


COPPER    LIQUOR. 


125 


so  that  the  experiment  was  now  definitely  interrupted. 
The  solution  contained  2  grams  of  copper  and  10 
grams  of  arsenic  per  liter. 

These   results   can  be   best   understood   from    the 
following  tabular  statement : 


CURRENT 
DENSITY  PER 
SQ.  METER. 

71  Amp. 
?6  " 
18  " 


PRESSURE. 


'•95 

1.85 
1.69 


V. 


CURRENT 
EFFICIENCY. 

97  % 


THE  FINAL  LIQUOR. 

6     g.  Cu  -)-  10  g.  As  per  L. 

3-2  g.    "     "   "        "     "    " 

2       rr     "      ««          «      •  •      * » 


It  may  be  again  mentioned  that  these  numbers 
relate  to  liquors  which  were  well  agitated  during  the 
electrolysis. 

Assuming  that  the  preceding  results  have  been 
confirmed  by  several  controls,  steps  can  then  be 
taken  for  the  calculation  of  the  plant.  The  lower 
the  current  density  selected,  the  more  completely  can 
the  liquor  be  freed  of  copper.  The  current  density 
of  1 8  amperes,  tested  last,  is  certainly  not  to  be  recom- 
mended. It  gave  only  4  grams  more  of  copper  than 
the  current  density  71,  but  to  do  this  it  required  an 
electrode  surface  four  times  as  large,  hence  a  larger 
plant,  and  the  efficiency  also  was  about  5  per  cent, 
less.  The  middle  current  density,  that  of  36  am- 
peres, is  more  favorable.  This,  or  in  round  numbers 
40  amperes,  may  be  made  the  basis  of  the  following 
calculation,  taking  the  pressure  as  1.9  V.  and  the  final 
liquor  as  containing  4  grams  of  copper  per  liter. 
According  to  these  conditions,  the  liquor  at  first  con- 


126        ELECTROCHEMICAL  EXPERIMENTS. 

tains  40  grams  of  copper  per  liter,  and  is  to  be 
worked  down  to  4  grams  of  copper  per  liter,  hence 
for  each  liter  or  each  cubic  meter  there  should  be 
obtained  36  grams  or  36  kg.  of  copper  respectively. 
The  working  of  the  daily  production  of  5  cb.  m. 
should  then  yield  180  kg.  of  copper  per  day.  Since 
the  current  efficiency  is  96  per  cent.,  there  would  be 
deposited  for  each  ampere  hour,  not  the  theoretical 

i.i 8 1  grams,  but  only  I.i8l  X  ~  ~  =  I-I33  grams  of 

copper.  Consequently,  the  precipitation  of  180  kg. 
of  copper  would  require  a  quantity  of  electricity 

180,000 
equal   to =  158870   ampere   hours.     As    the 

electrolysis  demands  a  pressure  of  1.9  V.,  therefore 
the  work  would  be  158,870  X  1.9  =  =  301,853  volt- 
ampere  hours.  This  calculated  into  horse-power 

hours  is  — L——  =  410.1  H.  P.  hours.  If  twenty-four 
hours  are  to  be  worked  per  day,  the  machine  must 
have  -  =  17.1  H.  P.  The  efficiency  of  a  17 

horse-power  dynamo  can  be  considered  equal  to 
about  90  per  cent.,  so  that  eventually  the  power  re- 
quired for  the  work  will  be  found  equal  to  — 

0.90 

19.0  H.  P. 

In  calculating  the  size  of  dynamo  necessary  for  the 
above  purpose,  it  may  be  assumed  that  there  is 
a  daily  consumption  of  301,853  volt-ampere  hours. 


COPPER    LIQUOR.  I2/ 

This  would  require  in  24  hours  a  steady  output  of 
-  =  12,577  V.  amperes.     As  I  bath  requires  1.9 

V.,  the  work  projected  could  be  done  with  a  machine 
of  1.9  V.  and  6620  amperes  (as  1.9  X  6620  =  12578). 
Two  baths,  in  series,  would  require  2  X  1.9  =  3.8  V., 

and  -—  =  3310  amperes.     Four  baths,  with  similar 

3-8 
arrangement,   would   demand:    pressure   =   4.19  = 

7.6 V., and  current  strength:  — —  =    1655  amperes, 

etc. 

The  greater  the  number  of  baths  arranged  in  series, 
the  greater  must  the  pressure  of  the  machine  be,  but 
the  current  strength  can  be  correspondingly  low. 
The  current  strengths  mentioned  above  are  too  high 
for  a  machine.  It  is  better,  as  already  mentioned  on 
p.  72,  not  to  exceed  500  amperes.  In  using  a  current 
of  300  amperes  the  corresponding  pressure  would  be 

=  41 .9  V.,  and  with  this  pressure  it  would  be  pos- 

4!  .Q 

sible  to  arrange  — -  =  22  baths  in  series.     With  this 

relatively  simple  working  of  baths  twenty-two  will  not 
be  too  many,  and  the  current  strength  will  not  be  too 
high,  therefore  a  machine  of  the  model  42  V.  AND  300 
AMP.  can  be  selected  for  the  work.  It  is  always  well 
to  allow  a  certain  latitude  in  such  matters  ;  therefore 
it  is  best  to  consider  propositions  for  a  machine 
which,  with  the  lowest  velocity  possible,  will  yield  a 


128        ELECTROCHEMICAL  EXPERIMENTS. 

current  of  300  amperes  with  a  pressure  of  42-45  V., 
but  it  must  still  be  possible  to  increase  it,  without 
danger,  to  350  amperes. 

After  the  energy  consumption  for  the  process  has 
been  calculated  as  19  H.  P.  and  the  dimensions  of  the 
machine  are  of  such  proportions  as  to  give  42  V.  and 
300  amperes,  the  size  of  plant  can  be  next  considered 
— i.  e.,  the  number  and  size  of  baths  and  the  number 
and  size  of  electrodes.  The  number  of  baths  to  be 

42 
arranged  in  series  is  • —  =  22.    As  the  current  density 

must  equal  40  amperes  per  sq.  m.  there  should  be 

—  =  7.C  sq.  m.  of  electrode  surface  in  each  bath.  A 
40 

suitable  and  convenient  size  for  the  electrodes  is  50 
X  50  cm.  As  both  sides  of  the  electrodes  are  used, 
the  surface  becomes  0.5  sq.  m.  Therefore,  to  get  7.5 
sq.  m.  surface  for  each  bath  it  will  be  necessary  to  use 

-  =  15   such  plates.     The   lead  plates,  serving  as 

anodes,  are  of  equal  size;  in  number  they  are  16,  or  I 
more  than  the  kathode  strips,  this  is  because  an  anode 
should  be  suspended  opposite  the  reverse  side  of  the 
last  kathode  plate.  The  lead  plates  should  be  about  2 
mm.  thick  and  the  copper  plates  0.3  mm.  in  thickness. 
It  will  be  necessary  to  purchase  the  copper  plates 
only  in  the  initial  experiment,  because  as  the  work 
progresses  these  will  be  produced  in  the  process. 
If  the  kathode  surfaces  be  covered  with  an  exceed- 


COPPER    LIQUOR. 


I29 


ingly  thin  layer  of  fat,  a  little  thicker  on  the  edges, 
they  soon  become  coated  with  layers  of  copper, 
which  can  readily  be  removed  in  sheet  form. 


FIG.  24. 


The  electrodes  are  hung  parallel  in  the  bath,  with 
alternating  anodes  and  kathodes,  the  copper  plates 
being  connected  with  one  another,  and  the  plates  of 


I3O       ELECTROCHEMICAL  EXPERIMENTS. 

lead  in  like  manner.  The  current  is  carried  to  the 
adjacent  bath  by  the  device  a  or  b,  Fig.  24. 

Directions  as  to  the  manner  of  fastening  the  elec- 
trodes, etc.,  will  not  be  here  given.  They  can  be 
found  in  the  various  volumes  of  Berg-  u.  Huttenman- 
nischen  Zeitung,  as  well  as  in  Balling's  Grundriss  der 
Electrometalhirgie^  Stuttgart,  1888. 

Dimensions  of  the  Bath.  In  the  preliminary  experi- 
ments the  electrodes  were  removed  3  cm.  from  each 
other.  This  separation  is  somewhat  too  small.  It 
would  be  better  to  make  the  distance  between  them 
from  4  to  5  cm.,  so  that  by  the  buckling  of  the  elec- 
trodes beneath  the  liquid  surface  short  circuits  may 
not  arise.  It  is  especially  at  the  beginning  of  the  ex- 
periment, when  the  copper  sheets  are  still  thin,  that 
this  danger  exists.  Later,  when  they  have  become 
thicker  and  firmer,  it  is  less  to  be  feared.  When  an 
agitator  is  used,  it  may  diminish  short  circuiting,  but 
in  spite  of  this  the  separation  of  the  electrodes  3  cm. 
from  each  other  can  still  be  made  the  basis  of  cal- 
culation. Thirty-one  electrodes  give  30  intermediate 
places  —  90  cm.,  and  the  distance  of  each  terminal 
electrode  from  the  side  of  the  trough  being  4  cm., 
there  would  in  all  be  a  full  length  of  98  cm.  The  full 
width  of  the  trough,  granting  equal  separation  of 
electrodes,  would  be  4  -f-  50  -f-  4=  58  cm.  To  pre- 
vent the  lead  peroxide,  which  slowly  disintegrates, 
from  becoming  incorporated  with  the  copper  plate,  the 
latter  should  not  be  allowed  to  reach  entirely  to  the 


COPPER    LIQUOR.  13! 

bottom  of  the  vessel.  Its  distance  from  the  bottom 
should  be  4  cm.  Further,  the  trough  must  never  be 
filled  to  overflowing.  A  spare  of  7  cm.  should  exist 
between  the  edge  and  the  liquid  surface.  The  depth, 
consequently,  of  the  trough  would  then  be  4  -f  50 
-f-  7  =  61  cm.,  and  the  depth  of  liquor  in  it  would  be 
54  cm.  The  full  size  of  trough  should,  therefore,  be 
in  length,  breadth,  and  depth  98  X  56  X  61  cm.  The 
volume  of  liquor  would  then  be  9.8  X  5.8  X  5.4  = 
307  liters.  The  volume  of  the  electrodes  would 
cause  a  reduction  of  10  L.,  leaving  in  all  297  liters. 

Should  the  dimensions  of  the  bath,  using  the  pre- 
ceding calculation  as  basis,  prove  too  large,  it  can 
be  divided  into  two,  or  several,  smaller  baths. 
These  should  be  arranged  parallel,  and  the  group 
that  results  can  then  be  introduced,  in  series,  as  one 
single,  large  bath. 

An  interesting  question  is,  In  what  time  will  the  con- 
tents of  the  bath  be  exhausted  ?  As  300  amperes  are 
active  in  the  bath  and  1.133  grams  of  copper  are  pre- 
cipitated with  the.observed  current  efficiency  of  96  per 
cent,  per  ampere  hour,  there  would  result  an  hourly 
yield,  from  the  bath,  of  300  X  1.133  —  34°  grams  of 
copper.  A  liter  of  liquor  would  yield  40  —  4  =  36 
grams  of  copper,  while  the  contents  of  the  trough,  con- 
taining 297  L.,  would  give,  consequently,  36  X  297  = 
10,692  grams  of  copper.  The  time  requisite  for  this 

would   be  -      -  =  31^  hours.      At  the  end  of  this 
340 


132        ELECTROCHEMICAL  EXPERIMENTS. 

period  the  entire  22  troughs  would  have  to  be  re- 
plenished. This  would  require  22  X  297  =  6534  liters 
of  liquor.  During  this  period  there  have  also  been  pro- 
duced 5  X  — —  —  6.563  cb.m.  of  liquor.  The  calcu- 

24 

lation,  therefore,  agrees.  The  working  of  the  liquor 
keeps  pace  with  its  production  during  the  operation, 
but  care  should  be  had  for  a  receptacle,  in  which  can 
be  collected  the  liquors  obtained  from  two  days' 
operation. 

The  example  may  be  amplified  by  calculating  the 
daily  production  of  the  plant  on  the  basis  that  copper 
is  precipitated  in  22  baths  arranged  in  series,  and 
that  the  dynamo  yields  300  amperes  with  42  V. 
Such  are  the  statements  which  generally  reach  the 
public,  while  the  actual  process  is  held  as  secret. 
Let  us  see  how,  from  such  statements,  the  daily 
production,  at  least,  can  be  calculated.  The  pressure 
need  not  be  taken  into  account  in  this  calculation ;  it 
is  sufficient  for  the  energy  consumption.  The  current 
strength  and  the  number  of  baths  arranged  in  series, 
are,  however,  important.  With  22  baths  thus  ar- 
ranged and  a  current  strength  of  300  amperes,  300 
amperes  would  be  active  in  each  of  these  baths,  or  a 
total  of  22  X  300  =  6600.  It  is  quite  immaterial 
whether  the  22  baths  are  simple,  or  whether  22 
groups  of  2,  3,  or  4  baths  each,  in  series,  are  worked. 
The  only  difference  being  that  with  22  simple  baths 
300  amperes  fall  upon  a  single  bath,  whereas  with  22 


COPPER    LIQUOR.  133 

quadruple  baths  300  amperes  will  fall  to  one  quadruple 
bath,  —  a  group,  —  so  that  a  single  bath  will  take  — 

=  75  amperes.  But  in  the  entire  circuit  there  will  be 
a  total  of  22  X  300  =  6600  amperes  doing  actual  work. 
For  each  ampere  hour  there  is  a  theoretical  produc- 
tion of  1.181  grams  of  copper,  while  the  entire  plant 
would  yield,  consequently,  22  X  300  X  1.181  per 
hour,  and  22  X  300  X  1.181  X  24  daily,  which 
equals  187,070  grams  of  copper.  This  number  must 
yet  be  multiplied  by  the  current  efficiency.  In  the 
preceding  example  it  was  known  to  be  96  per  cent., 
so  that  the  daily  production  would  be  179,587  = 


Instead  of  attempting  to  free  the  liquor  from  cop- 
per in  a  single  operation,  it  may  be  worked  up  in 
two  stages,  at  first  with  greater  current  density, 
reducing  this  toward  the  end,  or  the  attempt  may  be 
made  to  arrange  the  process  for  continuous  work. 
This  can  be  effected  by  letting  the  liquor  slowly 
enter  the  first  bath,  then  conducting  it  into  the  next, 
and  so  on  until  it  flows  from  the  last  bath  fully 
deprived  of  its  copper.  The  first  baths,  containing 
rich  liquors,  may  have,  under  certain  conditions, 
greater  current  densities,  consequently  less  electrode 
surface  than  the  end  baths.  These  statements  merely 
aim  to  show  the  possibility  of  reaching  the  desired 
goal  by  various  methods. 

The   few  grams   of  copper  per  liter,  remaining  in 


134  ELECTROCHEMICAL    EXPERIMENTS. 

the  final  liquors,  are  removed  by  hydrogen  sulphide, 
by  cementation,  or  by  any  other  good  method. 

The  evolution  of  oxygen  during  the  electrolysis 
causes  a  disagreeable  spattering  of  the  liquor.  This 
may  be  much  diminished  by  swimming  a  sheet  of 
oiled  paper  on  the  bath.  Agitation  of  the  liquor  is 
then  out  of  the  question,  but  it  can  be  mixed  by  cir- 
culation or  by  similar  means. 


6.  ARRANGEMENT    FOR    ELECTROCHEMICAL 
ANALYSIS. 

As  a  rule,  the  pressure  needed  in  analyses  by  elec- 
trolysis does  not  exceed  4  V.,  therefore,  we  may  use, 
as  sources  of  electric  energy,  either  4  large  Daniell 
cells,  arranged  in  series,  or  a  thermopile  (largest 
model  with  4  V.),  or  two  secondary  batteries  arranged 
in  series.  The  decided  advantage  of  electrochemical 
analysis,  its  extreme  accuracy,  as  well  as  the  cir- 
cumstance that  the  work  in  the  main  is  self-acting, 
leaving  the  chemist  free  to  perform  other  tasks,  are 
recognized  on  all  sides.  As  a  consequence,  electroly- 
sis is  pushing  its  way  more  and  more  into  technical 
establishments.  In  these  places  many  analyses  are 
conducted  simultaneously,  hence  the  source  of  the 
energy  must  yield  a  rather  powerful  current  and  still 
have  low  internal  resistance.  The  three  arrange- 
ments previously  described  render  this  possible. 
Meidinger  cells  are  not  well  adapted  for  the  work,  as 


ELECTROCHEMICAL    ANALYSIS.      .  135 

they  have  great  internal  resistance.  The  writer, 
using  a  battery  consisting  of  4  large  Daniell  cells  (22 
cm.  high),  conducted  simultaneously  5  copper  deter- 
minations, or  two  nickel  estimations,  and  with  two 
secondary  batteries  made  simultaneously  12  elec- 
trolytic determinations  of  the  most  varied  character. 
To  render  each  electrolysis  independent  of  the  rest,  as 
far  as  regards  current  strength,  it  is  only  necessary  to 
insert  between  each  experiment  and  the  main  current 
a  suitable  regulating  resistance.  This  is  most  simply 
made  as  follows  (Fig.  25):  Two  conducting  strips, 
in  the  form  of  flat  wire,  are  run  from  the  battery,  on 
the  wall,  along  the  experiment  table.  One  of  the  two 
electrode  stands  is  connected  directly  with  one  of  the 
metallic  conducting  strips.while  between  the  other  con- 
ducting strip  and  the  second  stand-support  a  resistance 
wire  is  inserted  (see  p.  117  and  Fig.  23).  The  brass 
clip,  serving  as  a  movable  contact,  is  held  firmly  by 
pushing  a  short  piece  of  rubber  over  it.  The  author 
uses  for  this  purpose  1.5  m.  of  rheotan  wire,  0.4  mm. 
in  thickness,  which  represents  5.5  U.  If,  occasion- 
ally, greater  resistance  is  necessary,  connect  the 
stands  by  the  conducting  metal  strips,  not  with  cop- 
per wire,  but  by  means  of  a  spiral  of  German  silver. 
By  pushing  the  movable  contact  back  and  forth  the 
current  strength  can  be  sufficiently  regulated.  It 
may  be  determined  by  inserting,  during  the  experi- 
ment, a  measuring  instrument  between  one  of  the 
stands  and  the  main  current.  The  amperemeter  of 


136 


ELECTROCHEMICAL  EXPERIMENTS. 


Kohlrausch  *   (Fig.    n)   answers   admirably  for  this 
purpose. 

The  customary  electrodes  are  of  platinum.     A  few 


FIG.  25. 


words  may  be  mentioned    in    regard   to  their  form. 
The  electrodes,  originally  proposed  by  Luckow  and 


*  Compare  Classen,  Quantitative  Analyse  durch  Electrolyse.     3te 
Auflage,  S.  52  ;  also  Smith,  Electrochemical  Analysis,  second  edition. 


ELECTROCHEMICAL    ANALYSIS.  137 

later  introduced  into  the  Mansfield  works,  consisted 
of  a  closed  cylinder  or  cone.  This  was  used  as 
kathode,  while  the  anode  was  a  platinum  wire.  Alex. 
Classen,  who  has  rendered  marked  service  in  the 
extension  of  electrolysis,  particularly  recommends 
(loc.  cit.)  platinum  dishes. 

The  writer  does  not  favor  this  idea,  because  the 
dish  allows  of  no  other  interruption  of  the  analysis 
than  to  siphon  out  the  liquid  contained  in  it,  water 
being  poured  in  at  the  same  time  to  replace  that 
which  was  removed.  If  the  liquid  remaining  from 
the  electrolysis  is  to  be  still  used  in  the  determination 
of  other  metals,  nothing  further  remains  than  to 
again  concentrate  the  diluted  liquid  to  a  suitable 
volume.  In  scientific  work  this  is  not  a  very  disturb- 
ing factor,  but  in  practical  operations,  where  the 
question  of  "  time "  represents  an  expensive  factor, 
the  plan  can  not  be  adopted.  By  using  cylindrical  or 
cone-shaped  electrodes  the  siphoning  can  easily  be 
avoided.  The  electrodes  should  have  side  slits  and 
should  not  be  suspended  from  below  in  the  stands, 
but  from  above  (Fig.  26). 

When  the  analysis  is  finished,  loosen  the  screw 
with  the  left  hand,  pressing  the  electrode  with  the 
right  to  the  stand.  Raise  it  vertically,  taking  care 
not  to  touch  the  anode  spiral.  The  moment  the 
edge  of  the  beaker  glass  is  passed,  the  little  liquid 
adhering  to  the  edge  is  removed  and  the  electrode 
quickly  immersed  in  a  glass  containing  water.  The 
J 


133 


ELECTROCHEMICAL  EXPERIMENTS. 


anode  is  then  immediately  removed.  It  is  washed  with 
water  from  a  wash  bottle.  The  electrodes  after  washing 
with  water  are  dipped  into  alcohol  and  dried  directly 
over  a  flame.  This  procedure  will  perhaps  cause  a 
slight  loss  in  liquid,  but  it  is  so  slight  that  in  techni- 
cal operations  it  need  not  be  at  all  considered.  The 
loss  equals  about  0.5-0.8  c.cm. ;  consequently,  with  a 
residual  liquid  of  150  c.cm.  it  would  be  0.3-0.5  per 


FIG.  26. 


cent,  of  the  remaining  metals.     For  example,  in  the 
case  of  a  sample  of  German  silver  containing  20  per 

20  0.3 


cent.  Ni,  the  error  would  be 


—  0.06  per 


100         100 

cent.  Ni.  In  contrast  with  this  slight  inaccuracy, 
falling  within  the  limit  of  error  of  other  methods,  we 
have  the  saving  of  time,  which  is  very  appreciable 
and  of  importance. 

The  platinum  cylinders  and  cones  should  have  a 
number  of   perforations,  so  that  the  lines   of   force 


ELECTROCHEMICAL    ANALYSIS.  139 

arising  at  the  central  anode  can  exert  themselves  on 
the  outer  surface  of  the  kathode.  If  this  is  neglected, 
the  major  portion  of  the  precipitate  will  be  found  on 
the  inner  kathode  surface,  especially  when  there  is 
not  much  liquid  between  the  electrodes  and  the  glass 
vessel.  Since  much  depends,  in  the  separation  of 
some  metals,  on  the  current  density,  perforation  of  the 
kathode  surface  is  to  be  recommended,  so  that  little 
difference  will  exist  between  the  current  density  upon 
the  inner  and  outer  surface.  A  wire  is  usually  made 
to  serve  as  the  anode.  It  is  bent  in  the  most  various 
forms.  Any  one  in  practice  who  would  bend  the 
anode  into  fantastic  shapes  would  justly  arouse 
astonishment.  In  scientific  laboratories  this  is  ap- 
parently a  matter  of  no  moment,  because  we  find  it  to 
be  almost  universally  the  case  that  the  bending  of  the 
anode  rarely  accords  with  the  kathode.  The  two 
electrodes  should  be  equidistant  at  all  points,  there- 
fore a  cylindrical  anode  is  by  far  the  most  satisfactory 
when  using  a  cylinder.  When  a  cone  is  used,  the 
bent  wire  should  have  the  form  of  a  cone-shaped 
screw  (Fig.  26).  The  anode  wires  should  always 
stand  or  rest  on  the  bottom  of  the  beaker  glass,  be- 
cause the  gas  evolved  about  them  will  then  effect  the 
mixing  of  the  solution;  consequently,  the  lower  liquid 
layer  would  stagnate  did  not  the  anode  extend  to  the 
bottom  of  the  beaker.  The  forms  of  electrodes  just 
described  and  the  manner  of  work  just  mentioned, 
have  been  used  and  followed  by  the  writer  for  years, 


I4O        ELECTROCHEMICAL  EXPERIMENTS. 

and  in  several  thousand  electrolytic  determinations 
have  answered  excellently.  They  fulfill  all  practical 
demands. 

In  communicating  an  electrolytic  method  for  the 
determination  or  separation  of  metals,  the  nature  of 
the  solution,  the  approximate  concentration,  but  espe- 
cially the  current  density  should  be  mentioned.  Cylin- 
drical or  conical  electrodes  must  be  considered  with 
both  surfaces.  If  there  be  references  with  the  reverse 
statement  in  literature,  then  introduce  an  ampere- 
meter between  the  electrolytic  stand  and  the  main 
current,  and  by  means  of  the  resistance-strip,  or  with 
a  rheostat,  produce  the  current  strength  which  has 
been  calculated  from  the  given  current  density  and 
the  size  of  the  electrodes  in  use.  In  this  way  alone 
is  it  possible  to  use  the  observations  of  others,  with- 
out necessitating  a  minute  copying  of  the  details  of 
the  first  author.  Having  once  ascertained  for  a  new 
process  how  much  resistance  must  be  thrown  in  with 
the  arrangements  at  hand,  in  order  to  arrive  at  the 
prescribed  current  density,  it  will  not  be  absolutely 
necessary,  in  later  trials,  to  insert  the  amperemeter  in 
the  circuit.  It  will  suffice  to  use  the  resistance  of 
the  amperemeter  as  found. 


TABLES. 


141 


G.  TABLES. 
I. 

ELEMENT. 

SYMBOL 

AND 

VALENCE. 

ATOMIC 
MASS. 

QUANTITY  DE- 
POSITED PER 
AMPERE  HOUR. 

Aluminium, 

Al'" 
Sb"' 

As"' 
Ba" 
Bi'" 
Br' 
<M" 
Ca" 
Cl' 
Cr'" 
Co" 
Cu" 
Cu' 
FT 
Au'" 
H' 
If 
Fe" 
Pb" 
Li' 
Mg" 
Mn" 
Hg" 
Hg> 
Ni" 
N'" 
O" 

ptiv 

K' 

Ag' 
Na' 
Sr" 
S" 
Sn" 
Zn" 

27.04 
119.6 

74-9 
136.86 
207.5 
79.76 
111.70 
39-91 
35-37 
52-45 
58.6 
63.18 

19.06 
196.2 
i 
126.54 
55-«8 
206.39 
7.01 
23-94 
54-8 
199.8 

58.6 
14.01 
I5-96 
194-34 
3903 
107.66 
23.00 
87.30 
31-98 
JI7-35 
64.88 

0-337    gr- 
1.491 

0-934 

2-559 
2-587 
2-983 
2.088 
0.746 

1-323 
0.654 
1.096 
1.181 

2.363 
0.713 
2.446 
0.0374 
4-732 
1.045 
3-859 
0.262 
0.448 
1.025 
3.736 
7.472 
1.096 

o.i75 
0.298 
1.817 

1-459 
4.026 
0.860 
1.632 
0.598 
2.194 
1.213 

im 

Antimony, 

Arsenic,    
Barium,    
Bismuth,  . 

Bromine,  . 

Cadmium,     
Calcium,  
Chlorine,  .... 

Chromium,  .    .        

Cobalt,      

Copper,     
Fluorine,  . 

Gold,    . 

Hydrogen,    . 
Iodine,      

Iron,     ...        .        .... 

Lead, 

Lithium,  

Magnesium,     

Manganese, 

Mercury,  . 

Nickel,     

Nitrogen,     

Oxygen, 

Platinum, 

Potassium,    
Silver,  ... 

Sodium,    
Strontium,    

Sulphur,   

Tin, 

Zinc.      

142 


TABLES. 


II.  THERMOCHEMICAL,  DATA. 


(from  JVaumann's  Thermochemie.} 

HYDROGEN. 

ARSENIC. 

(H2,0) 

68360 

(As2,  0.) 

154590 

CHLORINE. 

(As2,O3,  aq) 
(As2,  06) 

147040 
219400 

(Cl,  H) 

22OOO 

(As2,  05,  aq) 

225400 

(Cl,  H,  aq) 

39320 

(C12,  05,aq) 

-  20480 

POTASSIUM. 

SULPHUR. 
(S,  O2) 
(S,  02,aq) 
(S02,  0) 
(S02,  0,  aq) 
(SO2  aq,  O) 

71070 

78770 
32160 
71330 
63630 

(K,0,  H) 
(K,0,  H,aq) 
(K,  S,  H,  aq) 
(K3,0,aq) 

IK,  ci) 

(K,  Cl,  aq) 
(K2,  O,  SO3aq) 

I  04000 
116460 
65100 
164560 
105610 
101170 
195850 

(  O.   O  0  ) 

103230 

(S03,  aq) 

39170 

SODIUM. 

(S,04,H2) 
(S,  04,H2,aq) 

192910 
210760 

(Na,  O,  H) 
(Na,  O,  H,  aq) 

102030 
111810 

(S,  H2) 

4510 

(Na,  S,  H,  aq) 

60450 

(S,  H2,aq) 

9260 

(Na2,  0,  aq) 

155260 

IODINE. 
(H,  I) 
(H,I,aq) 

—   6040 

(Na2,  O,  SO3aq) 
(Na,  O,  Cl,  aq) 
(Na,  Cl) 
(Na,  Cl,  aq) 

186640 
83310 
97690 
96510 

BROMINE. 
(Br,  H) 
(Br,  H,  aq) 

8440 
28380 

CALCIUM. 
(Ca,  0) 
(Ca,  0,  aq) 

131360 

149460 

NITROGEN. 

(Ca,  C12) 

170230 

(N,H3) 

11890 

(Ca,  C12,  aq) 

187640 

(N,  H3,  aq) 
(N2,0) 
(N,0) 
(N2,03,aq) 
(N,02) 
(N2,05,aq) 

20330 
-  18320 

—  21575 
—   6820 
2OO5 
29820 

STRONTIUM. 
(Sr,  0) 
(Sr,  O,  aq) 
(Sr,  C12) 
(Sr,  C12,  aq) 

i  30980 
157780 

18455° 
195690 

(N,03,H) 

4I5IO 

BARIUM. 

(N,  03,H,aq) 

49090 

(Ba,  O) 

130380 

(Ba,0,  aq) 

158260 

PHOSPHORUS 

(Ba,  C12) 

194250 

(P,04,H3,aq) 

305290 

(Ba,  C12,  aq) 

196320 

TABLES. 


MAGNESIUM. 

LEAD. 

(Mg,  0) 

145860   (Pb,  O) 

50300 

(Mg,  0,  H20) 

148960 

(Pb,  C12) 

82770 

(Mg,02,H2) 

217320 

(Pb,  Cl2,aq) 

75970 

(Mg,  C12) 

151010 

(Pb,  0,  S03aq) 

73800 

(Mg,  Cl2,aq) 

186930 

(Pb,  0,  N205aq) 

68070 

(Mg,  0,  S03aq) 

180180 

(Pb,  S) 

20400 

ALUMINIUM. 

COPPER. 

(A12,  C16) 

321870 

(Cu2,  0) 

40810 

(A12,  C16,  aq) 
(A12,  03,  3S03aq). 

47556o 
45  '770 

(Cu,  0) 
(Cu2,Cl2) 

37160 

65750 

MANGANESE. 

(Cu,  C12) 
(Cu,  C12,  aq) 

51630 
62710 

(Mn,  C12) 
(Mn,  C12,  aq) 
(Mn,  O,  H20) 
(Mn,  02,  H20) 
(Mn,  0,  S03aq) 

111990 
128000 
94770 
116280 
121250 

(Cu,  O,  SO3aq) 
(Cu,  O,  N2O5aq) 
(Cu2,S)  ' 
CADMIUM. 
(Cd,  C12) 

5596o 
52410 
20240 

93240 

ZINC. 

(Cd,Cl2,aq) 

96250 

(Zn,  O) 

85430 

(Cd,  O,  SO3aq) 

89500 

(Zn,  O,  H2O) 

82680 

SILVER 

(Zn,  C12) 
(Zn,  C12,  aq) 
(Zn,  O,  SO3aq) 
(Zn,  S) 

N^ICKKI 

97210 
112840 
106090 
41989 

(Ag2,  0) 
(Ag,  Cl) 
(Ag,  Br) 
(Ag,  I) 

5900 
29380 
22700 
13800 

(Ni,0,  H20) 
(Ni,  C12) 
(Ni,Cl2,  aq) 

60840 
74530 
93700 

(Ag2,  0,  N205aq) 
(Ag2,  0,  S03aq) 
(Ag2,  S) 

16780 
20390 
53io 

(Ni,  0,  S03aq) 

86950 

MKRCURY. 

OoBALX 

(Hg2,  0) 

42200 

(Co,0,  H20) 
(Co,Cl2) 
(Co,  Cl,,  aq) 
(Co,  0,  S03aq) 

6^400 
76480 
94820 
88070 

(Hg,  0) 
(Hg2,Cl2) 
(Hg,  C12) 
(Hg,  C12,  aq) 

30660 
82550 
63160 
59860 

IRON. 
(Fe,Cl2) 
(Fe,Cl2,  aq) 
(Fe2,Cl6) 
(Fe2,Cl6,aq) 

82050 

9995° 
192060 
255420 

TIN. 
(Sn,  C12) 
(Sn,  C12,  aq) 
(Sn,  C14) 
(Sn,  C14,  aq) 

80790 
81140 
127240 
157160 

(2Fe,  C12  aq,  C12) 

55520 

GOLD. 

(Fe,  0,  S03aq) 
(Fe2,  03,  3S03aq) 

93200 
224880 

(Au,  C13) 
(Au,  C13,  aq) 

22820 
27270 

(Fe,  S) 

355°4 

(Au,  C13,  HC1  aq^ 

31800 

III.  WIRE  RESISTANCES. 


DIAMETER. 

CROSS- 
SECTION. 

RESISTANCE  PER  i  M.  OF   WIRE. 

Nickelin. 
li 

Rheotan. 
O 

Copper. 

O.IO 
0.15 
O.2O 
0.25 

0.008 
0.018 
0.031 
0.049 

51 

22 

13 

8 

60 
26 
15 

9-5 

2.23 
0.99 
0.56 
0.36 

0.30 

0-35 
O.4O 

0-45 
0.50 

0.071 
0.096 
0.126 
0.159 
0.196 

5-6 
4-1 
3-2 

2-5 

2  O 

6-7 
4-9 
3-7 
2.9 

2.4 

0.247 
0.182 
0.139 
O.I  10 

0.089 

0-55 

o  60 
0.65 
0.70 

0.75 

0.238 
0.283 
0.332 
0-385 
0.442 

1.68 
1.41 
i.  20 
1.04 
0.90 

1.99 
1.67 
1.42 

1-23 
1.07 

0.074 
0.062 

0.053 
0.045 

0.040 

0.80 
0.85 

0.90 
o-95 

I.O 

0-503 
0.568 
0.636 
0.709 
0.785 

0-79 
0.70 

0.63 
0.56 
0.51 

0.94 
0.83 
0.74 
0.66 
0.60 

0.035 

0.031 
0.028 
0.025 

O.O22 

.2 

•3 
•4 

•5 

0.950 
I.I31 

1.328 

1-539 
1.767 

0.42 

0-35 
0.30 
0.26 
0.23 

0.50 
0.42 

0-35 
0.31 
0.27 

0.018 

0.016 
0.013 

O.OII 
O.OIO 

.6 

•  7 
.8 

•9 

2.O 

2.009 
2.270 
2-545 
2-835 
3-!4i 

0.199 
0.176 

0-157 
0.141 
0.127 

0.235 
0.208 
o.i  86 
o.  167 
o.  150 

0.009 
0.008 
0.007 
0.0062 
0.0056 

2.1 

2.2 
2-3 
2-4 

2-5 

3-464 
3.801 

4.155 
4-524 
4.909 

o.  1  15 

0.105 
0.096 
0.088 
0081 

0.137 

0.124 
0.114 
0.105 
0.096 

0.0051 
0.0046 
0.0043 
0.0039 
0.0036 

2.6 

2.7 

2.8 

2.9 

3-o 

5-309 
5-725 
6.158 
6.605 
7.069 

0.075 
0.070 
0.065 
0061 

^7- 

0.089 
0.082 

0.077 

0.072 

_    0^)67 

0.0033 
0.0031 

0.0028 
0.0026 
0.0025 

I 


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